“Mix 500 grammes of Cologne glue with three sheets of strong white blotting-paper, or four sheets of white tissue paper, shredded as small as possible, and boil until it becomes thick, stirring the whole into a perfectly smooth pulp. Let it boil thoroughly, and while stirring continually, and working with a stout wooden rod, add 21⁄2kilogrammes of very finely sifted dry purified whiting. After working this mixture thoroughly, add 80 grammes of linseed oil, which must be also thoroughly worked in. To preserve the glue add 50 grammes of Venetian turpentine. This stone cement will take any shade of colour if mixed with lamp-black or coloured earths.”
“Mix 500 grammes of Cologne glue with three sheets of strong white blotting-paper, or four sheets of white tissue paper, shredded as small as possible, and boil until it becomes thick, stirring the whole into a perfectly smooth pulp. Let it boil thoroughly, and while stirring continually, and working with a stout wooden rod, add 21⁄2kilogrammes of very finely sifted dry purified whiting. After working this mixture thoroughly, add 80 grammes of linseed oil, which must be also thoroughly worked in. To preserve the glue add 50 grammes of Venetian turpentine. This stone cement will take any shade of colour if mixed with lamp-black or coloured earths.”
The various methods for the preservation of iron objects which have been or are still in use may be divided into two groups. To the one group belong those methods in which the objects are preserved with their coating of rust, or with the rust that has penetrated them; to the other group belong those in which the removal of rust precedes preservation. The former methods must be applied when the iron has been completely converted into rust or when the rust has only left a small metallic core. These methods may of course be used also for all iron antiquities.
The methods of the second group can be applied to those objects only which still retain a strong metallic core, in which case the objects regain the more or less grey or white surface of fresh unoxidized iron. These methods are at present little known, and therefore but little used, for owners and the general public are still accustomed to see in the covering of rust the evidence of antiquity with which they are loth to part.
In addition to these methods, there are others which are of an intermediate kind, either special or a combination of methods from both these groups.
(1)Methods of preserving Objects of Iron without removal of the Rust.
Impregnation.The earliest processes, which are to some extent still in use in some collections, are simple impregnation methods, in which the object is either painted once or more with the impregnating medium by means of a brush, or is placed directly in the medium itself. In either case the penetrating power of the solution used is directly proportional to its fluidity.
The following media may be used for the purpose:
(1) Warm size.(2) Warm isinglass solution.(3) Solution of waterglass.(4) Solution of shellac in alcohol.(5) Rubber solution in carbon bisulphide. The mass after swelling is dissolved in benzine[110].(6) Copal varnish diluted with turpentine.(7) Copal varnish mixed with linseed oil[111].(8) Linseed oil.(9) Linseed varnish.(10) Linseed varnish mixed with an equal quantity of petroleum.(11) Bees’-wax dissolved in turpentine.(12) Bees’-wax dissolved in benzine.(13) Petroleum.(14) Vaseline.(15) Melted paraffin.(16) Oleate of lead: 100 grammes of olive oil, 100 gr. of lead oxide, and 100 gr. of water are boiled until all the water has evaporated and the mass has become grey. The mass is extracted by shaking it with alcohol, and the residue is dissolved in absolute ether, in the proportion of 100 gr. of ether to 5 gr. of the substance. Before use it should be diluted with a little ether[112].(17) Speerschneider’s mixture. This consists of 8 parts of rape oil, 1 part of bees’-wax, 1 part of pine resin, and 2 parts of benzene[113].(18) Collodion, or the mixture used in the Museum at Donaueschingen, which consists of 30 grammes of collodion, 2 gr. of camphor, and 1 gr. of oxalic ether.
(1) Warm size.
(2) Warm isinglass solution.
(3) Solution of waterglass.
(4) Solution of shellac in alcohol.
(5) Rubber solution in carbon bisulphide. The mass after swelling is dissolved in benzine[110].
(6) Copal varnish diluted with turpentine.
(7) Copal varnish mixed with linseed oil[111].
(8) Linseed oil.
(9) Linseed varnish.
(10) Linseed varnish mixed with an equal quantity of petroleum.
(11) Bees’-wax dissolved in turpentine.
(12) Bees’-wax dissolved in benzine.
(13) Petroleum.
(14) Vaseline.
(15) Melted paraffin.
(16) Oleate of lead: 100 grammes of olive oil, 100 gr. of lead oxide, and 100 gr. of water are boiled until all the water has evaporated and the mass has become grey. The mass is extracted by shaking it with alcohol, and the residue is dissolved in absolute ether, in the proportion of 100 gr. of ether to 5 gr. of the substance. Before use it should be diluted with a little ether[112].
(17) Speerschneider’s mixture. This consists of 8 parts of rape oil, 1 part of bees’-wax, 1 part of pine resin, and 2 parts of benzene[113].
(18) Collodion, or the mixture used in the Museum at Donaueschingen, which consists of 30 grammes of collodion, 2 gr. of camphor, and 1 gr. of oxalic ether.
In addition to these materials, there are other mixtures of resin, varnishes, and bees’-wax, with their appropriate solvents, but they do not possess any special advantages as impregnating solutions.
After treatment with size or isinglass, iron objects may be given when dry a coating of linseed oil, linseed varnish, solution of shellac, etc.
The materials numbered 7 to 10 in the above list should be applied warm to enable the viscid fluids to penetrate the rust, for the more readily the solution enters the object the better is the result obtained. Apart from the fact that they are easily ignited at a high temperature, they must not be heated beyond 230°F. [110°C.], otherwise objects which consist largely of rust will fall to pieces[114].
In the process of impregnation a twofold result is aimed at, viz. to prevent the rust from crumbling, and to exclude air from the specimen. The application of heated linseed oil or linseed varnish is founded upon the supposition that these substances enter into a chemical combination with ferric oxide to form a stable compound; this is, however, disputed by some modern authorities[115]. Neutral substances offer a safer method for the exclusion of air, and of these melted paraffin is undoubtedly the best. The paraffin must be quite pure andfree from stearine, as can be ascertained from the melting point; thus pure paraffin melts at 130°-150°F. [55°-65°C.], stearine at 160°F. [70°C.]. Paraffin with a melting-point higher than 65°C. should be looked upon with suspicion.
In many collections the objects are heated before impregnation with media which are insoluble in water, or they are exposed to the air for six to twelve months after excavation. This latter proceeding is, however, certainly inadvisable if the iron contains chlorine, and if this is the case not one of these methods produces satisfactory results.
On the other hand, as almost all the iron antiquities which do not contain chlorine compounds may be treated by the methods of the second group, simple and direct impregnation is passing more and more out of use. Before impregnation all soluble substances, especially chlorine compounds, must be removed by steeping.
(2)Preservation by Steeping and Subsequent Impregnation.
Krause’s Method.The water used for steeping should be preferably lukewarm, and should be changed every twenty-four hours. It is even better, at least for the first time, to lay the object in water and then raise it to boiling-point, a measure which will allow the more ready penetration of the water. As in the case of limestones, earthenware, etc. (p.59), care must be taken to place the objects as near to the surface of the water as is possible. Small objects may be put in glass jars, large ones in wooden troughs, tin vessels, or wooden boxes lined with zinc or lead. Any little excrescences on the iron, which are frequently filled with ferrous chloride, should be punctured to give the water unimpeded and more speedy access. Crumbling objects should be held together by tightly wrapping them in muslin. Curators must decide forthemselves how far means such as files, chisels, or small hammers may be used to remove the rust or earthy material conglomerated by rust.
Although much recommended, the method of adding soda or lime water to remove the chlorine as soluble sodium chloride or calcium chloride is, in our opinion, inadvisable. Both these substances precipitate the iron from the ferrous chloride or ferric chloride (which are soluble in water) as insoluble hydroxide of iron, which more or less closes the interstices, and thus impedes the access of water to the interior.
The process of steeping can here again be controlled by the use of the silver solution (p.62), for if there no longer appears any or only very little cloudiness the steeping may be considered complete. The length of time required for steeping depends upon the thickness of the rust and the porosity or existence of cracks in it, and if the objects are of considerable size, it may extend over several weeks.
After steeping the object should either be dried in the open air, and later on a warm stove, or be placed for a few days in alcohol to remove the water, after which the rapid evaporation of the alcohol will quickly dry it. The steeping of iron objects in warm alcohol has been recommended[116], but if their size is considerable the method is an expensive one. This method has the advantage that the alcohol penetrates the rust sooner than does water, and also prevents oxidation, which may be actually produced by the water. It may perhaps be advisable to dilute the alcohol, the usual strength of which is 95% to 96%, with about an equal volume of water, for some salts are not readily soluble in pure alcohol. When dry the object is warmed for a few hours in a mixture of equal parts of good linseed varnish and petroleum. The petroleum serves to dilute the varnish, which canthus more quickly permeate the entire mass of iron and rust. On account of the inflammable nature of the mixture the warming should be done over a water-bath. For small objects a cylinder made of ordinary tin-plate, measuring from 6 to 10 inches [15 to 25 cm.] in diameter and 6 in. [15 cm.] in height, may be used. To increase stability the lower half should be of a smaller diameter, and fitted into an iron tripod. The same end is attained by soldering a ring round the middle of the cylinder, which will rest on the ring of the tripod. The cover consists of a number of copper rings gradually diminishing in diameter, which fit closely into one another, thus enabling porcelain vessels of various sizes to be used. For larger objects, such as swords, two long rectangular troughs (Fig.24) of stronger plate should be used. The following sizes will probably be found useful: one about 40 inches [100 cm.] long by 4 inches [10 cm.] broad, and 4 inches [10 cm.] deep, and the other slightly larger. Handles should be fixed at the upper edges. Three iron bars 1 inch [21⁄2cm.] thick and 4 inches [10 cm.] in length are laid across the bottom of the larger trough, on which the smaller is placed. The space between the two vessels is filled with water to a depth of 2 inches [6 cm.]. The trough is warmed on a stove, or better, where gas can be had, by means of a number ofBunsen burners fitted with rose or ring burners, over which the trough may be supported upon tripods. While heating care must be taken that the water does not boil over, which can be easily avoided by regulating the gas supply. As the water evaporates further quantities should be added as required. After simmering for about two hours, the objects should be removed and allowed to drain; they should then be placed on a tripod, or on glass rings, on the warm stove in cold weather, to accelerate the evaporation of the petroleum and the setting of the varnish. In summer drying chambers may be used; these are sold by dealers in physical and chemical apparatus, or can be made at little cost by a tinsmith.
Fig. 24.Water-bath.1⁄15nat. size.
Fig. 24.Water-bath.1⁄15nat. size.
If the objects have been steeped in pure alcohol, or at least towards the end of the treatment in three changes of alcohol, so that all the water is replaced by alcohol, they may be dipped directly without drying in the varnish mixture, for the alcohol evaporates in the varnish bath which is at a temperature of 194°-203°F. [90°-95°C.]. As the varnish hardens, the iron thus treated acquires a glazed surface; other means of impregnation may therefore appear preferable, e.g. a solution of gum-dammar or melted paraffin. For impregnation with the dammar solution the object must first be dried, and the air-pump used in the way described on p.68. On account of the inflammable nature of the benzine heat must not be applied, nor indeed is it necessary.
When impregnating with pure paraffin[117], specimens may be lightly wiped with a cloth, but need not be dried. The paraffin may be heated to 212°-248°F. [100°-120°C.] without danger, so long as it is kept from direct contact with the flame. A thermometer should be used, and, as soon as the paraffin has melted at a temperature of about 60°C., theobject should be placed in it by means of tongs. When the temperature has risen above 212°F. [100°C.] the water is converted into steam, and causes a brisk ebullition of the melted paraffin. The quantity of paraffin used should, therefore, be such that its level remains at the least 2 inches [5 cm.] below the upper edge of the vessel.
When the bubbles have ceased to rise, thus showing that all the water is expelled, the paraffin should be allowed to cool to a temperature of 180°-190°F. [80°-90°C.]. The iron should be taken out with tongs, and the liquid allowed to run off. It should then be wrapped, while still at 80°C., in soft blotting-paper or in a piece of old linen to absorb the superfluous paraffin. If the surface of the object is very uneven, or if there are deep cracks or holes in which the paraffin can collect, it will, when cold, form a white mass, and should therefore, while still warm and fluid, be soaked up with filter paper, or distributed evenly by means of suitable brushes. The superfluous paraffin may also be absorbed by putting the object in dry sawdust; any sawdust which remains attached can be removed when cold with benzine, or it may be scraped from the spots where it has collected with a knife or spatula. Any spots where the iron may have become exposed may be covered with a thin coat of paraffin dissolved in benzine.
Ekhoff’s Method[118]. The objects are laid for two or three months in water which is changed every two or three days, a small quantity of quicklime being added[119]. After this steeping, and after some of the rust has been removed mechanically, the object is lightly dried and put into heavy petroleum of sp. gr. 0·85 to 0·95, which is then heated up to 220°F. [105°C.]. A thermometer should be used to ensure this temperature. This temperature being higher than the boiling-point of water,the water contained in the object evaporates and causes the petroleum to bubble, as in the method previously described. When all the water has been replaced by the petroleum the bubbling ceases. After the fluid has somewhat cooled down, the iron is taken out and is allowed to remain for about an hour in sawdust, which absorbs the superfluous oil. Finally, while gently warming the object over a warm, but not too hot, stove, it is coated over with a mixture of 1 part of bees’-wax and 2 parts of turpentine, or better with paraffin dissolved in benzine. Heavy petroleum, which we have found by experience to be a suitable material, is preferable to varnish in so far as the iron is impregnated by a neutral substance which is practically liquid paraffin, but has the disadvantage of being highly inflammable and of being difficult to obtain at so high a specific gravity.
Straberger’s Method.This method, for the description of which I am indebted to Herr Straberger, has proved effective in the preservation of a number of iron antiquities in the Museum at Linz on the Danube. Even iron objects, which had been in bad condition and had undoubtedly contained chlorine, have after treatment by this method shown no signs of change, while the dull black surface has an agreeable appearance.
Straberger places the newly-excavated objects immediately into linseed oil to prevent the access of air. After remaining in the oil for some time they are taken out, wrapped in cloths saturated with linseed oil, and removed packed in sawdust. Upon arrival they are unwrapped and put into water, to which a small quantity of soda is added to remove the oil more easily. The water is frequently changed, and the objects are meanwhile cleaned mechanically with emery paper and hard brushes. Any blisters are removed by the aid of a small hammer and chisel. After steeping they are dried and smoked over a candle flame which is allowed to play over the wholesurface. The soot is then rubbed off with a cloth or soft brush. Objects with a smooth surface may be rubbed with india-rubber. The preservative action of this proceeding depends upon the fact that during the smoking, in addition to the soot, oily products of combustion are deposited from the candle flame, which prevent the access of air and moisture to the iron.
“Objects which are much decayed or cracked should, when cleaned and thoroughly dry, be again placed into linseed oil which has been slightly warmed and should remain therein for a few days before being smoked. Upon removal from this second oil bath they should be lightly wiped and dried over a moderately warm stove or in the sun. Patience is necessary, and nothing further should be done until the oil has entirely dried in the fine cracks and crevices and firmly binds the mass. The oil crust on the surface is then loosened by soaking in a strong soda solution and wiped off, after which the object is dried, smoked over the candle flame, and the soot wiped or brushed off with a soft brush. The smoking and wiping may be repeated if necessary.”
“Objects which are much decayed or cracked should, when cleaned and thoroughly dry, be again placed into linseed oil which has been slightly warmed and should remain therein for a few days before being smoked. Upon removal from this second oil bath they should be lightly wiped and dried over a moderately warm stove or in the sun. Patience is necessary, and nothing further should be done until the oil has entirely dried in the fine cracks and crevices and firmly binds the mass. The oil crust on the surface is then loosened by soaking in a strong soda solution and wiped off, after which the object is dried, smoked over the candle flame, and the soot wiped or brushed off with a soft brush. The smoking and wiping may be repeated if necessary.”
Herr Straberger states that his treatment has been successful when impregnation with isinglass and coating with shellac has failed.
The methods of Hartwich and Jacobi hold an intermediate place between the above methods and those which will be subsequently explained. With the former they have this in common that they do not call for the entire removal of the rust and that they require the use of linseed oil; on the other hand their application presupposes the existence of a strong metallic core, otherwise when the rust is removed they will show merely a skeleton of the original object. The existence of a sufficiently substantial metallic core can be easily ascertainedfrom the weight, for an object, which consists solely or in great part of the oxide of a metal, is much lighter than one of the same size which is largely metal. The ring also affords a test, for an iron object, of which the greatest part is metallic iron, gives a clearer note when struck than one which is chiefly rust. A still more certain test is the use of a file or a drill (comp. page107).
Hartwich’s Method[120]. This method is intended for objects of an especially large size, the hard oxide coating of which does not allow satisfactory steeping. Hartwich heats the object to redness, allows it to cool slowly, and then scrapes off the outer layer which has been rendered friable by this treatment. The subsequent procedure is that of Krause’s method, viz. warming in linseed varnish.
Jacobi’s Method.The method of preservation of iron antiquities used in the Saalburg Museum at Homburg is described by Jacobi as follows: The object is heated in the fire of a forge, which causes the chief part of the rust to flake off, while any rust which still adheres is removed when cold by water and brushing. The object is again held in the flame with tongs and heated (smaller objects may be placed on an iron plate); and during the heating is quickly taken out three or four times and each time brushed over with linseed oil. Most of the linseed oil is thus burnt and the deposition of carbon gives to the iron a black colour, while the oil which has been partially burnt or hardened by the heat produces a slight lustre. This process, as carried out at Homburg by a locksmith, is that which blacksmiths ordinarily use to blacken iron objects and to protect them from rust. The preservation has proved permanent, and only in rare cases has it been found necessary to repeat the process. These good results are probably due to the fact that the antiquities of iron preserved in that Museum are for the most part found ingood condition, having very little rust and certainly containing only a very small amount of chlorine. Iron articles which contain chlorine but which still have a good metal core, after washing, drying, and a cautious preliminary application of heat, are ready for treatment by Jacobi’s method.
Inlaid Iron Objectsrequire especially cautious treatment. Although I have not had any personal experience in the treatment of objects of this kind, good results have been obtained in several Museums, especially in that at Mainz.
The following quotation from the “Merkbuch” (p. 75) describes the method which is applied at Mainz, where it probably originated:
“Objects of this kind which are likely to have been originally inlaid with silver, gold, copper or brass, as is frequently the case with objects of the Merovingian period, are not placed in alcohol after the steeping, but are warmed and dipped three or four times into a hot dilute solution of isinglass. The heating is necessary, otherwise the isinglass will set on the surface and will not penetrate into the interior. When the object has been dried and the isinglass has set, the layer of rust which covers the inlaid ornaments is scraped off with a graving tool, and any spongy hollow parts are filled up with a paste made of iron rust and isinglass, before the inlaid work is cleaned. During the scraping the object is held in the left hand on a little wooden board covered with plush or thick chamois leather, to which it is fixed as firmly as is necessary by means of a vice. In scraping special care must be taken that the graving tool follows the lines of the designs, for in scraping across the design it may slip under the flat silver thread and raise it out of its place. When the ornamentation has been completely laid bare, it is rubbed with emery cloth and then polished with a brush and fine emery powder. Thepiece is then dipped into a solution of gum-dammar, and, when the surface is dry, emery is again used to remove the varnish, which gives the silver a slightly yellow colour. The object is then protected from the influence of air and moisture by the transparent retouching varnish of Sohnée frères (Paris).”
“Objects of this kind which are likely to have been originally inlaid with silver, gold, copper or brass, as is frequently the case with objects of the Merovingian period, are not placed in alcohol after the steeping, but are warmed and dipped three or four times into a hot dilute solution of isinglass. The heating is necessary, otherwise the isinglass will set on the surface and will not penetrate into the interior. When the object has been dried and the isinglass has set, the layer of rust which covers the inlaid ornaments is scraped off with a graving tool, and any spongy hollow parts are filled up with a paste made of iron rust and isinglass, before the inlaid work is cleaned. During the scraping the object is held in the left hand on a little wooden board covered with plush or thick chamois leather, to which it is fixed as firmly as is necessary by means of a vice. In scraping special care must be taken that the graving tool follows the lines of the designs, for in scraping across the design it may slip under the flat silver thread and raise it out of its place. When the ornamentation has been completely laid bare, it is rubbed with emery cloth and then polished with a brush and fine emery powder. Thepiece is then dipped into a solution of gum-dammar, and, when the surface is dry, emery is again used to remove the varnish, which gives the silver a slightly yellow colour. The object is then protected from the influence of air and moisture by the transparent retouching varnish of Sohnée frères (Paris).”
A modification of Krefting’s method (p.108) has proved eminently successful in the treatment of iron objects inlaid with silver. Krause[121]recommends that the article be placed, with the inlaid surface downwards, for 24 hours in a mixture of
10 grammes of 40% acetic acid,10 grammes of ammonium chloride,70 grammes of distilled water,10 grammes of aluminium powder.
10 grammes of 40% acetic acid,10 grammes of ammonium chloride,70 grammes of distilled water,10 grammes of aluminium powder.
It is then removed from the bath, carefully brushed and washed, and, if the inlaid work is not yet cleaned, is replaced in the bath. This is repeated until the inlaid work is completely exposed. Spots of ferroso-ferric oxide which are difficult to remove may be ground away by an emery wheel, care being taken that the inlaid surface is held against the lower side of the wheel (which must be rotated in the reverse direction) so that it is always in sight.
All the methods of this group, which have been applied to many articles in various Museums, exhibit one inherent defect, for any rust which remains after treatment may cause the continued oxidation of the iron. The effects of this action of rust are, I believe, extremely small, and it must at the same time be admitted that iron antiquities, even if they have been well steeped and afterwards impregnated, do not always remain in a permanent and sound state of preservation. Ifin such a case the well-known small watery bubbles should make their appearance, the steeping has undoubtedly been insufficient. This evil can be remedied by gradually heating the object to redness to destroy the impregnating material, and by a careful repetition of the steeping and impregnation.
(3)Preservation of Iron Antiquities by Removal of the Rust.
Steffensen’s Method (Copenhagen).The objects are carefully heated over a flame and are then laid in dilute sulphuric acid. The sulphuric acid dissolves a certain amount of the iron, and it is found by experience that the chemical action is strongest at those spots where any rust remains, and that this is detached by the hydrogen which is produced. When the cleaning is sufficient, the iron is laid in a dilute soda solution to neutralise the acid, and is afterwards well washed with water and dried in an oven. When dry the iron is brushed over with a solution of bees’-wax (or better of paraffin) in benzine, the evaporation of which leaves a protective coating of bees’-wax or paraffin.
Blell’s Method.The method proposed by Blell and applied by him to many of the objects in his collection is distinct from that described above, although in its earlier stages the principle is the same. The following quotation is taken from the description of his method which the author read before the Antiquarian Society[122]at Königsberg:
“If a specimen is found to have a sufficiently strong core of iron it should be heated in the furnace to bright redness and then dipped into water. The expansion of the iron caused by the heat and the subsequent contraction caused by the sudden cooling thoroughly loosens thelayer of rust. Large iron objects with a strong and firmly attached incrustation of rust will require a repetition of the process. By this means not only is the rust converted into a red powder which is easily rubbed off, but the object itself is rendered more suitable for the subsequent treatment. At the same time the heating process removes any coating of oil, fat, etc., which may have remained from previous attempts at preservation, and which would interfere with the further stages of the process. Smaller or delicate specimens should be treated in the flame of a spirit-lamp, but special care must be taken that there is sufficient iron present. Sword blades and other tools and weapons with sharp edges should be heated only, for the sudden cooling may cause cracks in the cutting edges.”
“If a specimen is found to have a sufficiently strong core of iron it should be heated in the furnace to bright redness and then dipped into water. The expansion of the iron caused by the heat and the subsequent contraction caused by the sudden cooling thoroughly loosens thelayer of rust. Large iron objects with a strong and firmly attached incrustation of rust will require a repetition of the process. By this means not only is the rust converted into a red powder which is easily rubbed off, but the object itself is rendered more suitable for the subsequent treatment. At the same time the heating process removes any coating of oil, fat, etc., which may have remained from previous attempts at preservation, and which would interfere with the further stages of the process. Smaller or delicate specimens should be treated in the flame of a spirit-lamp, but special care must be taken that there is sufficient iron present. Sword blades and other tools and weapons with sharp edges should be heated only, for the sudden cooling may cause cracks in the cutting edges.”
To complete the removal of the incrustation of rust which has been loosened by the heating process, or by the heating and sudden cooling, the object should be placed
“in a well-stirred mixture composed of one part by weight of sulphuric acid in nine parts of water. Bubbles of hydrogen will immediately rise and the rust will begin to separate. In freshly prepared acid objects which are not very rusty will be freed from rust after four to six hours, those covered with a deeper layer of rust in about twelve hours, but several days, or even weeks, may be necessary. The duration of the process depends upon the strength of the acid and the character of the rust, viz. whether it is thick and solid, or thin and porous, and whether the iron is of a soft, or of a hard character.When first making use of this method it is advisable to use dilute acid and to take out the objects several times in the course of the day and examine them, whileduring the night they should be taken out of the acid and placed in soft water[123].For the acid bath and for rinsing it will be found convenient to have two pairs of wooden troughs having the following internal measurements:(1) An internal length of 10 inches [25 cm.] by 71⁄2inches [19 cm.] in breadth and 43⁄4inches [12 cm.] in depth, which will be useful for the larger number of objects.(2) For long narrow objects, e.g. sword-blades, and long spear-heads, the internal measurements should be 40 inches [100 cm.] long by 4 inches [10 cm.] broad and 3 inches [8 cm.] deep.Small fragile objects are most satisfactorily treated in glass vessels or glazed earthen pots or vases.The acid must have free access to all parts of the object; if a sword, for example, lies flat upon the bottom, the under-surface apparently remains unacted upon by the acid. This should be remedied by the use of a couple of small wooden supports.Frequent rubbing with a cloth and forge scale[124]or coarse sand greatly helps in removing the rust, but gentler treatment is required for the smaller and more fragile objects. The rust is often very firmly attached in some portions of the object, and in this case those areas which have been already freed from rust should be coated over with lard, which is free from salt, to protect them from further action of the acid, while the pockets of rustare alternately treated with acid and graving tools. No particle of rust should be allowed to remain, for sooner or later it will begin to spread, whatever precautions may be taken.The action of the acid becomes less effective if it has been used for several objects. A little fresh acid should then be added. The more active the sulphuric acid, the brighter will be the grey colour of the iron after the rust has been removed. If old acid has been used the iron will be of a dirty grey colour, and should then be placed into fresh acid for a short time until it assumes a clear light grey colour.The third part of the process begins with the removal of the iron from the acid bath and has as its object the removal of every trace of the acid, otherwise the rust will very quickly return and cover the whole surface. The object is therefore immediately and repeatedly rinsed in soft water and carefully dried; the cheapest material for this purpose is cotton waste, but ordinary linen-cloth must be used for objects with jagged edges, for the threads will catch in the notches and hinder the drying. This should be done without delay, or a change of the colour from light grey to yellow will betoken a new formation of rust. Articles showing a very complicated construction, which are however rare from the Iron Age, should be packed in perfectly dry hot pinewood sawdust, while those which are still more difficult to dry, for example, coats of chain-mail, after thorough rinsing, should be immediately put into a pan with melted lard, free from salt, and boiled until the cessation of bubbling shows that all the water has been driven off by evaporation.They are then rubbed dry or are laid in hot sawdust, after which they are brushed over with melted lard andplaced in this condition for at least half an hour in a moderately hot cupboard until the fat has penetrated into the finest pores of the iron. That this has really taken place may be proved by the use of a file.When by this means all trace of sulphuric acid has been removed the fourth stage of the process is reached, viz. the removal of the grease from the surface and the subsequent application of some preparation to prevent the access of air and moisture. Most of the grease is removed by placing the objects in a warm place on blotting-paper. Any grease still remaining on the surface can be entirely removed with a cloth or paint-brush by means of benzine. If no restoration or repair is required nothing more is necessary than to apply the protecting solution.”
“in a well-stirred mixture composed of one part by weight of sulphuric acid in nine parts of water. Bubbles of hydrogen will immediately rise and the rust will begin to separate. In freshly prepared acid objects which are not very rusty will be freed from rust after four to six hours, those covered with a deeper layer of rust in about twelve hours, but several days, or even weeks, may be necessary. The duration of the process depends upon the strength of the acid and the character of the rust, viz. whether it is thick and solid, or thin and porous, and whether the iron is of a soft, or of a hard character.
When first making use of this method it is advisable to use dilute acid and to take out the objects several times in the course of the day and examine them, whileduring the night they should be taken out of the acid and placed in soft water[123].
For the acid bath and for rinsing it will be found convenient to have two pairs of wooden troughs having the following internal measurements:
(1) An internal length of 10 inches [25 cm.] by 71⁄2inches [19 cm.] in breadth and 43⁄4inches [12 cm.] in depth, which will be useful for the larger number of objects.(2) For long narrow objects, e.g. sword-blades, and long spear-heads, the internal measurements should be 40 inches [100 cm.] long by 4 inches [10 cm.] broad and 3 inches [8 cm.] deep.
(1) An internal length of 10 inches [25 cm.] by 71⁄2inches [19 cm.] in breadth and 43⁄4inches [12 cm.] in depth, which will be useful for the larger number of objects.
(2) For long narrow objects, e.g. sword-blades, and long spear-heads, the internal measurements should be 40 inches [100 cm.] long by 4 inches [10 cm.] broad and 3 inches [8 cm.] deep.
Small fragile objects are most satisfactorily treated in glass vessels or glazed earthen pots or vases.
The acid must have free access to all parts of the object; if a sword, for example, lies flat upon the bottom, the under-surface apparently remains unacted upon by the acid. This should be remedied by the use of a couple of small wooden supports.
Frequent rubbing with a cloth and forge scale[124]or coarse sand greatly helps in removing the rust, but gentler treatment is required for the smaller and more fragile objects. The rust is often very firmly attached in some portions of the object, and in this case those areas which have been already freed from rust should be coated over with lard, which is free from salt, to protect them from further action of the acid, while the pockets of rustare alternately treated with acid and graving tools. No particle of rust should be allowed to remain, for sooner or later it will begin to spread, whatever precautions may be taken.
The action of the acid becomes less effective if it has been used for several objects. A little fresh acid should then be added. The more active the sulphuric acid, the brighter will be the grey colour of the iron after the rust has been removed. If old acid has been used the iron will be of a dirty grey colour, and should then be placed into fresh acid for a short time until it assumes a clear light grey colour.
The third part of the process begins with the removal of the iron from the acid bath and has as its object the removal of every trace of the acid, otherwise the rust will very quickly return and cover the whole surface. The object is therefore immediately and repeatedly rinsed in soft water and carefully dried; the cheapest material for this purpose is cotton waste, but ordinary linen-cloth must be used for objects with jagged edges, for the threads will catch in the notches and hinder the drying. This should be done without delay, or a change of the colour from light grey to yellow will betoken a new formation of rust. Articles showing a very complicated construction, which are however rare from the Iron Age, should be packed in perfectly dry hot pinewood sawdust, while those which are still more difficult to dry, for example, coats of chain-mail, after thorough rinsing, should be immediately put into a pan with melted lard, free from salt, and boiled until the cessation of bubbling shows that all the water has been driven off by evaporation.
They are then rubbed dry or are laid in hot sawdust, after which they are brushed over with melted lard andplaced in this condition for at least half an hour in a moderately hot cupboard until the fat has penetrated into the finest pores of the iron. That this has really taken place may be proved by the use of a file.
When by this means all trace of sulphuric acid has been removed the fourth stage of the process is reached, viz. the removal of the grease from the surface and the subsequent application of some preparation to prevent the access of air and moisture. Most of the grease is removed by placing the objects in a warm place on blotting-paper. Any grease still remaining on the surface can be entirely removed with a cloth or paint-brush by means of benzine. If no restoration or repair is required nothing more is necessary than to apply the protecting solution.”
A white varnish has much to recommend it from its protective power, but as it gives to iron an unsatisfactory gloss, it is preferable to use a solution of bees’-wax in benzine.
Having made use of Blell’s method in a number of cases I have a few suggestions and modifications to offer. The heating should be carried out carefully and gradually, lest the sudden conversion of the moisture in the rust into steam should cause small explosions which would scatter pieces of rust. There is no danger of this if the objects are heated in an oven; they should not therefore be heated in an open flame. For smaller objects I use a box six inches [15 centimetres] square, of strong tin-plate loosely covered with an iron lid, or with a piece of asbestos sheet; but if the objects are large, e.g. swords, spearheads, etc., I heat them on a strong piece of tin-plate bent round to form a channel, and covered with a long piece of asbestos sheet, the edges of which are bent over the edges of the channel, to retain the heat as much as possible.
It is advisable, in my experience, to use the sulphuric acid well diluted, e.g. in the proportion of 1 to 20, and to renew it several times if necessary. In mixing concentrated sulphuric acid with water great caution is required on account of the evolution of heat. The acid should be poured in a thin stream into the water, but not vice versâ, and the mixture should be constantly stirred with a glass rod. If a glass vessel is used for the mixing, it must not be too thick lest the heat should cause it to break, but the larger the proportion of water to the sulphuric acid, the less considerable will be the rise of temperature.
For boring out rust spots which have eaten deeply into the iron a dental drill can be used with success, and a great variety of drills and milling cutters can be obtained. The rinsing, which Blell carries out by moving the object to and fro close under the surface in a vessel full of water, may be sufficient for thin iron objects, such as swords, knives, spear-heads, and similar objects. Larger specimens should be freed from the acid by putting them into a still more dilute solution, and, when necessary, by steeping for a short time in water. It may also be advisable to put the objects into dilute soda solution to neutralize the sulphuric acid, but this does not do away with the necessity for steeping in water. The brown coating of rust which may possibly follow the steeping can be removed by the use of steel-wire brushes, which can now be made of such fine wire that their softness almost equals that of a moderately soft tooth-brush. Brass-wire brushes should not be used, on account of the yellow colour which they give to the iron. I always put the objects directly after steeping into clean fat heated to 250°F. [120°C.], for brushing over with fat and warming in a stove often caused a slight tarnish to cover the surface. I have also used paraffin wax instead of fat.
For the method of restoring iron antiquities and of fillingup large gaps, the reader should refer to Blell’s detailed account; it will here suffice to quote his statement that a mixture of iron filings with tin filings can be used for this purpose. These are melted and applied by the aid of a blowpipe.
The accompanying illustrations represent iron antiquities which have been treated by Blell’s method: the sword (Fig.25) proved, after reduction, from its two ridges to be a scramasax; on the spear-head (Fig.26) treatment revealed a small copper ring at the most constricted part, while the fibula, which previously had been a mass of rust, now shows the spiral which had been totally disguised.
Fig. 25.Iron sword treated by Blell’s method.
Fig. 25.Iron sword treated by Blell’s method.
Fig. 26.Iron spear-head treated by Blell’s method.
Fig. 26.Iron spear-head treated by Blell’s method.
Fig. 27.Iron fibula treated by Blell’s method.
Fig. 27.Iron fibula treated by Blell’s method.
Krefting’s Method.The electro-chemical method of Krefting was originally published in “Aarsberetning fra Foreningen till Norske Fortidsmindesmaerkers Bevaring,” 1892 (p. 51), but in “Finska Fornminnesföreningens Tidskrift[125]” there is a translation into German by H. Appelgren, and an additional series of observations and experiments by him. His remarks are equally applicable to Blell’s method, and the following extracts and quotations from this paper give Krefting’s method of procedure and the circumstances under which it should be applied.
Small fragile objects such as fibulae, thin clasps and bracelets or those which are much eaten away by rust, are not suitable for this mode of treatment, thus:
“A knife which is much corroded, and which when taken out of the earth shows a distinctive form (for example, that of the Early Iron Age), may lose so much by the application of the electric current that every distinct sign of its original character is destroyed. The characteristic edges of a spear-head or of an axe of the late Iron Age, or the equally characteristic point of aniron sword, may, if the rust has eaten deeply into them, be unrecognisable when removed from the electrolytic bath. A sword, the hilt of which is inlaid with copper wire or is plated with silver or gold, or the blade inlaid with inscriptions in gold, silver, or copper, may be totally destroyed by incautious treatment; for the ornamentation, if undermined by rust, may be detached with the rust from the underlying iron.”
“A knife which is much corroded, and which when taken out of the earth shows a distinctive form (for example, that of the Early Iron Age), may lose so much by the application of the electric current that every distinct sign of its original character is destroyed. The characteristic edges of a spear-head or of an axe of the late Iron Age, or the equally characteristic point of aniron sword, may, if the rust has eaten deeply into them, be unrecognisable when removed from the electrolytic bath. A sword, the hilt of which is inlaid with copper wire or is plated with silver or gold, or the blade inlaid with inscriptions in gold, silver, or copper, may be totally destroyed by incautious treatment; for the ornamentation, if undermined by rust, may be detached with the rust from the underlying iron.”
On the other hand, objects of sound metallic iron covered with an incrustation of rust about1⁄25inch [1 millimètre] in thickness may be easily cleaned in this manner, but if on using a file the metal does not appear at all, or only at a depth of1⁄8inch [3 millimètres], great caution must be used. If there is reason to believe that there is gold or silver inlaid work undermined by rust, Appelgren recommends that the object should, as a preliminary, be laid in clean water, which should be renewed every day. After some time, three weeks at the most, sufficient rust will have been cleared away by carefully brushing with a steel brush to lay bare the ornamentation, at least in part, and it can then be ascertained whether there is any rust underneath which would, if Krefting’s method were used, cause the ornamentation to be detached.
The line of treatment is as follows: The metallic iron core is laid bare by filing in several places. The specimen is then wrapped with strips of zinc in such a way that the zinc is in actual contact with the bare metal (Fig.28). The whole is then placed into a 5% solution of caustic soda[126].Appelgren uses a solution of 31⁄2-41⁄2lbs. [11⁄2-2 kilogrammes] of caustic soda in 2 gallons [10 litres] of water. The rust is cleared away by voltaic action; the iron forms the negative pole, the zinc the positive of a voltaic cell, in which the water is resolved into its constituents, viz. oxygen and hydrogen. At the negative pole, i.e. the iron, the hydrogen rises up in small bubbles and acts in part by mechanically detaching the rust as in Blell’s method, in part also by the chemical conversion of the rust into metallic iron, or into a compound which contains a smaller quantity of oxygen than does ordinary rust. The oxygen combines with the zinc to form zinc oxide, which is dissolved in the soda solution. The process is usually completed in 24 hours[127]. The black powder which is loosely attached to the iron is best rubbed off with wet sand and fine wire brushes. Any hard pieces of black stable rust (Edelrost), magnetic oxide of iron, which have not yielded to theelectric current should be removed by means of a small chisel. After rinsing the object thoroughly in water, it should beplaced in melted paraffin at 240°F. [115°C.], which will expel every trace of moisture. On removal the melted paraffin should be allowed to drain off, and thus leave when cold a protective covering upon the iron[128].
The following points should be observed in the application of the method. Vessels of glass or glazed earthenware should be used for the reduction, while long swords can be put into tall glass cylinders or into wooden troughs, the interior of which must be coated over with paraffin. The soda solution must be kept in a closed glass bottle[129]. It should be diluted with water until the specific gravity, as shown by the hydrometer, is 1·06; the mixture will then contain about 5 per cent. of caustic soda. During the reduction process the mixture frequently assumes a brownish colour as the result of the presence of organic matter associated with the rust. On account of the dissolved zinc which it contains it cannot be used a second time, unless regenerated by boiling with quicklime. The solution is, however, so cheap that this is scarcely worth the trouble.
The objects should be handled with metal tongs, and should not be touched with the hand until they have at least been dipped or rinsed in water, for the soda solution has an injurious effect upon the skin. A basin containing vinegar, dilute hydrochloric or sulphuric acid should always be at hand into which the fingers should be quickly dipped if they have been in contact with the caustic soda. These materials will serve also for cleaning the vessels used in the reduction process.
The zinc strips should be1⁄4to1⁄3inch [1⁄2cm. to 1 cm.] in breadth, and should be cut out of a piece of sheet zinc of moderate thickness, but of sufficient pliability.
Any firmly fixed rust may be removed by mechanical means, such as the graver, drill, etc., as has been previously mentioned. If in rinsing a slight layer of oxide appears, although this is rare, it should be brushed off with a steel-wire brush.
If one portion only of a specimen requires reduction (the other portion having, for example, remains of wood attached, and therefore being unsuitable for reduction), that portion only should be wrapped with the zinc and immersed in the solution.
The results obtained by Krefting's preservation-process arequite as surprising as those which are afforded by Blell’s method. Figure29, taken from Appelgren’s work, shows the lower portion of a spear-head before and after treatment, by which it became apparent that the whole socket was plated with silver, with two engraved and gilded animal figures. Fig.34represents a piece of a sword, on which an inscription was brought to light by the reduction process.