Gum arabic25 grammesGrape sugar60 grammesPurified honey15 grammesAlcohol, 40 deg15 c.c.Water60 c.c.
Saturated solution of ammonium dichromate.
Two solutions to be mixed together before using in proportions 15 A, 25 B, 50 water.
Gum arabic60 grainsGlucose45 grainsGlycerine10 minimsPotassium dichromate30 grainsDistilled water2 ounces
Dextrine60 grainsWhite sugar75 grainsAmmonium dichromate30 grainsGlycerine2 to 8 minimsDistilled water3 ounces
The gum is first dissolved and the remainder of the ingredients added. It may be necessary to warm the solution in a hot water bath to dissolve it. It is then filtered through flannel or clean muslin, and preserved for use in well-stoppered bottles. With this solution clear glass plates are coated and dried by a gentle heat over a small spirit lamp. The plate while still warm is exposed under a reversed positive[C]for from two to five minutes in sunlight, and from 10 to 20 minutes in diffused light. The image is then but slightly visible. On removing from the printing frame the plate is laid in the air (protected from light) for a few minutes to absorb a little moisture from it. The next process is the "dusting on." If the image is required to be black, fine Siberian graphite is spread over it with a soft flat brush. This will adhere to the parts unaffected by light, giving an image of the positive. Any colored fine powder maybe used, giving images in various colors. When fully developed the excess of powder is dusted off and the film coated with collodion. After this it is well washed to remove the unaltered gum and dichromate salt. The film may, if desired, be detached from the plate and used for enamels, ivory, wood, textile fabrics, opals, etc.
Regular transparency gelatine dry plates are the handiest for making positives, especially for amateurs, if one does not care if the subject is in reverse, or if one has a reversed negative to work from. There is a "special" carbon tissue, price $4.00 per roll of 2 × 12 feet, made by the Autotype Company, of London, England, with full instructions appended; by a system of double transfer, reversed negatives may be obtained with this tissue. The "special" tissue is only to be used for the transparency. A safe edge of black paper is required on the transparency, pasted up exactly to the edge of the picture, on the glass side; it comes, sold in strips, gummed, ready for use, about ¼ inch wide; this is required, as the tissue used for the negative resist on the copper plate, which is printed from the transparency, must have a safe edge, shielded from the light, or it will not attach itself to the copper plate, the tissue coming inside half way. The screw pressure printing frame should have a piece of heavy felt for backing the transparency.
The following instructions for making carbon transparencies will no doubt be found useful:
The carbon tissue prepared for this process consists of paper coated with gelatine containing carbon, lamp-black, or other pigments.
The Autotype Company, of London, manufacture a special "transparency" tissue.
Cutting up the Tissueis performed by unrolling it gently upon a zinc cutting plate, cut square and true, with the inches marked at the bottom and right-hand side. By using a Tsquare and observing the numbered inches marked on the plate, it will not be difficult to cut the tissue to any dimension. If the tissue is very curly and unmanageable it should be kept down with convenient weights. After cutting it up to the required sizes, which should be conveniently smaller than the dish to be used for sensitizing, it should be kept flat under a metal plate.
Sensitizing the Tissueis the next operation. This is performed in a solution of potassium dichromate rendered alkaline with ammonia. Tie over the mouth of a two-gallon jug a piece of muslin, to form a kind of bag, into which place fifteen ounces of potassium dichromate, then fill up the jug with water and allow it to stand until the dichromate is dissolved and the solution becomes cold. It is sometimes advisable to regulate the quantity of dichromate. In hot weather, or for very thin negatives, the proportion of water should be doubled, while for very hard negatives only half the quantity should be used. In very hot weather it is often advantageous to replace about 30 per cent. of the water with the same quantity of alcohol.
The operation of sensitizing the tissue must be carried on in a room lighted by a window covered with a yellow blind. A flat dish of porcelain, glass, orpapier maché, a squeegee, and a sheet of glass or zinc larger than the tissue, will be required.
The solution is poured into the dish, and should be at least two inches deep. The tissue is then immersed in it, and the air-bells that form immediately brush away from both sides with a broad camel's-hair brush. The temperature of the bath should not be higher than 60 deg. Fahr.; and the time of immersion should be from three to five minutes. After the tissue has remained in the solution for the allotted time it is gently removed and laid face downward upon the glass or zinc plate, and the back squeegeed, removing all superfluoussolution. The tissue is removed from the glass and laid over a sheet of cardboard, bent into the form of an arch, to dry.
Another method (H. J. Burton's) of sensitizing carbon tissue is to lay it flat on a sheet of clean blotting paper, and sponge on the back a very strong sensitizing solution composed as follows:
Potassium dichromate4 ouncesLiquid ammonia fort1 ounceWater20 ounces
First mix the ammonia with the water, then grind up and add the dichromate.
Drying the Tissueshould be accomplished in a room perfectly free from the noxious fumes of other chemicals, and lighted only by non-actinic light. Tissues sensitized during the evening should be dry on the following morning. It should then be cut to the sizes required and kept flat in a pressure frame, or other similar contrivance.
Exposing the Tissue.—The tissue can be exposed behind the negative in an ordinary printing frame, or in special frames having no joint in the back, as no image is visible. The negative must be furnished with a safe edge, made by painting an edge about one-eighth of an inch round the negative with black varnish, or by pasting on strips of red or black paper. Exposure must be judged by an actinometer. A very suitable instrument for timing the exposure of carbon tissue is Sawyer's actinometer. It consists of a rectangular tin box with a glass lid, bearing twelve tints graduated from slight discoloration to a degree of opacity, representing the extreme amount of deposit upon the lights of the densest negatives, each division of this screen of tints bearing a number in opaque pigments; and a roll of sensitive paper is placed in the box, and the end pulled forward so as to pass under the tints. When this arrangement is placed in the light, the silver paper commences to discolor underneath the graduatedscreen, beginning of course at the lightest, but the number on the tint being in an opaque pigment is preserved white, and serves to register the progress of printing; for if, when the lid is opened, the number one, for instance, shows clearly on a tinted ground, the instrument is said to have registered one tint; by that time the number two will have begun to make its appearance, and, if sufficient exposure be given, the light will print through the whole scale by successive steps, and show up the numbers, one to twelve. With an instrument of this kind it is evident that, by exposing alongside the carbon tissue and determining the number of tints required for the proper exposure of that negative, the same number of tints with the same negative will always prove right. A little practice will enable one to judge the number of tints required for every class of negative.
It will be well to remark here that freshly sensitized tissue will produce inferior pictures to that used a day or two after; the pictures are not so hard, and there is less danger of the high-lights being washed away.
Continuing Action of Light.—If the carbon tissue after exposure to the light, be kept in the dark for a little time the effect on the print will be precisely the same as if the exposure to light had been prolonged. This continuing action of light may often be utilized to advantage. Pictures known to be under-exposed will, if kept till morning, by that time have acquired the same force as if they had received the proper exposure.
Developmentconsists simply in dissolving the gelatine unaffected by light, with hot water as the solvent.
Immerse the exposed tissue in a bath containing cold water. It will first of all curl up, but afterward lay flat and limp. It is then placed in another bath containing cold water together with a sheet of glass which has previously been coated with a 5 per cent. solution of gelatine. Bringthem together face to face, draw them out, and force into close contact with a large squeegee; then place between blotting paper for five or ten minutes. In squeegeeing, the tissue should be uppermost, and a sheet of American cloth laid over it to prevent the squeegee from damaging it.
Development should not be attempted for at least twenty minutes, during which time the glass, with the tissue on it, should be placed between sheets of blotting paper, and kept under pressure to insure its adherence to the glass support. After that time it is placed in a dish, and water heated to a temperature of 100 deg. F. added. The colored pigment will at once commence to ooze out of the edges, and after a little time the paper originally holding the carbon film may be removed with the hand. Then, by gently leveling the picture with the hand, the superfluous gelatine will be washed away, and if the exposure has been correct a perfect image should remain. A certain amount of control can be kept over an autotype picture. An over-exposed print will show itself by insolubility of the gelatine, and the high light refusing to be washed clear. The temperature should be raised considerably, and hot water poured over with a jug. If this fails to reduce the intensity, add a little ammonia to the water as a last resource, though the better plan is to make another print, giving less exposure. Under-exposure results in over-solubility of the gelatine. The half-tones will be washed clean away. It is rarely an under-exposed print can be saved. All that can be done is to reduce the temperature of the water. Development should never be hurried; the slower it is the better the gradation of tone in the results.
After development is complete the bichromate salt is discharged, and the image rendered perfectly insoluble by well washing in cold water and placing in a dish containing a 5 per cent. solution of potash alum, after which it is again washed and dried.
Another method of making a transparency and one that involves less trouble is by means of the transparency plates which are now in the market. Of these we have tried Carbutt's with the greatest success. For these the following instructions are given:
The requisites are, a deep printing-frame a size larger than the negative to be used, with a flat glass bottom clear and free from scratches (crystal plate is best), a dark-room Lantern, or other artificial light, and Keystone Gelatino-Albumen Plates. Transparencies can be made same size of negative by contact and exposure to artificial light, or enlarged or reduced in the camera by daylight, with equal perfection in result. To make transparencies by contact place one of the Keystone thin crystal glass transparency plates over the negative in printing-frame, lay piece of dark soft material over it, close down the back, and expose to the light of the lantern or to a gas flame or other artificial light, for 10 to 30 seconds, according to density of negative, at a distance of 20 inches from the flame. Use the following developer:
A.Metric Weight.Avoirdupois Weight.600 c.c.mDistilled Water20 ounces120 grammesSulphite of Soda Crystals4 ounces22 grammesEikonogen330 grains10½ grammesHydrochinon160 grains960 c.c.mWater to make up to32 ouncesB.Metric Weight.Avoirdupois Weight.600 c.c.mDistilled Water20 ounces60 grammesCarbonate of Potash2 ounces60 grammesCarbonate Soda Crystals2 ounces960 c.c.mWater to make up to32 ounces
For use take 1 ounce (30 c.c.) of A, ¾ ounce (25 c.c.) of B, with 4 ounces (120 c.c.) of water.
More of A will increase density, more of B will increase detail and softness. Temperature of developer should not vary much below 65 deg. nor above 75 deg. The after treatment is same as with any other developer.
Let the development continue until the blacks look quite strong, and detail showing in the high-lights; wash off developer, then immerse in
4 c.c.mSulphuric Acid1 drachm480 grammesHyposulphite of Soda16 ounces60 grammesSulphite of Soda2 ounces30 grammes[D]Chrome Alum1 ounce1920 c.c.mWarm Water64 ounces
Dissolve the hyposulphite of soda in 48 ounces (1440 c.c.m.) of water, the sulphite of soda in 6 ounces (180 c.c.m.) of water; mix the sulphuric acid with two ounces (60 c.c.m.) of water, and pour slowly into the sulphite soda solution, and add to the hyposulphite; then dissolve the chrome alum in 8 ounces (240 c.c.m.) of water and add to the bulk of solution, and the bath is ready. This fixing bath will not discolor until after long usage, and both clears up the shadows of the negative and hardens the film at the same time.
Let remain two or three minutes after transparency is cleared of all appearance of silver bromide. Then wash in running water for not less than half an hour to free from any trace of hypo solution. Swab the surface with wad of wet cotton, rinse, and place in rack to dry spontaneously. Then varnish with plain collodion.
The carbon tissue used as a resist, which is mounted on the copper plate, is made by the Autotype Company, London, England. No. 100 Standard Brown is the right grade to use, though I have reached good results with No. 103. The No. 100 is a heavier grade than No. 103, and requires two or three minutes longer exposure than the latter. Use a deep printing frame with a screw pressure to secure absolute contact, which is known by iridescent markings appearing on the glass of the printing frame. A Johnson's actinometer is very useful to time the exposure. From 4 to 6 tints are necessary. Experience here is the only guide, as the light varies as well as the density of the negative and the sensitiveness of the tissue. If one does not have an actinometer, a slip of albumen paper may be used; as soon as the paper has reached the darkest point, which is then called one tint, extend it so that a fresh portion comes out to the light, and so on for the different tints. In September, for instance, the darkest tint is reached in about 3 to 4 minutes; two tints and a half or 8 minutes in the shade at midday on a clear day in September is about right,—this is understood to be with medium negatives and No. 103 tissue sensitized within three days.
You should over-expose rather than under-expose, allowance being made when the acid is used. Print deeply, so that, on development, the negative tissue on the copper plate shows all the detail clearly in the shadows. The tissue should not appear very dark on the plate. The copper should show up through the gelatine clearly and brightly. The thinner the negative tissue, the quicker the biting of the acid.
The carbon tissue comes only in rolls of 2½ feet by 12 feet, price $3.00, not cut. It is not sold in a sensitive condition. Full instructions with each roll for sensitizing. Tap water will do, but I would suggest distilled water for making the sensitizing solution of bichromate of potassium.
Bichromate of potassium1 ounceWater16 ouncesAlcohol½ drachmAmmonia12 drops
The best way to sensitize the tissue, is to place the tissue face up, keeping it flat so that the solution reaches all parts at once, removing all air bubbles, and rubbing in the solution with the fingers until pliant; the time of immersion is three minutes in winter, two minutes in summer. The hands should be washed directly after handling the solution, and care must be taken that there are no cuts on the fingers, as the solution is very harmful, but if due care is exercised and the hands well washed immediately with soap, little, if any, trouble will be experienced; use rubber finger tips as much as possible. Keep the temperature of the solution at 70 deg. both in summer and winter. Take a piece of glass free from scratches (an inch larger all round than the tissue); have the glass ready cleaned with ammonia and talcum powder of fine whiting, squeegee the sensitized carbon tissue directly from the solution on to the glass and place to dry at night in a light-tight box; it will be dry in the morning.
The tissue is in the best condition for three days after sensitizing; it can be used up to seven days; it gradually increases in sensitiveness from day to day. After a week or ten days has elapsed, it is hard to manage and becomes unreliable. When the tissue is dry take a sharp knife and cut inside the edge, and strip off one corner. Fairly good results can be reached by simply drying over a curved piece of pulpboard, which is tied with string on each end, but squeegeeing on glass gives a sharper result. The addition of ammonia and alcohol to the sensitizing solution makes it easier to strip the paper from the copper plate, after the carbon tissue is mounted on the plate, and enables one to develop the resist in the water at a lower temperature than without it, thus avoiding pits in the darker portions and white specks or bubbles in the lights, should the water reach too high a temperature.
The best copper is recognized by its rosy lustre. Pure copper only should be used. It can be purchased ready polished and beveled from several firms in New York. The best way, if large quantities of plates are required, is to buy the copper in the rough of one firm and have it polished by another, and bevel it yourself if necessary with a file and burnish it by hand, or the firm who polish it will do the beveling for you.
The Scovill & Adams Co. supply copper-plates of the finest quality, ready polished, for photogravure work.
Total cost by this method about one-third less than by purchasing ready made. It makes a copper plate 1/16 grade, 9 × 11, cost about $1.10. Order your copper 1/16 in. grade up to 10 × 12; larger sizes 1/8 in. grade. If you use 1/16 in. grade above this size, the plate is liable to buckle. Be sure the plate is free from pits and scratches and with a high polish. Have what the polishers and engravers call a rouge polish. If they do not supply it, rouge it yourself with powdered rouge and turpentine, using a ball of absorbent cotton over a large piece of smooth cork. A good way to buy rouge is in the stick; it is more economical. Rub the wet cotton on it and the right quantity is assured. Pits in the copper may be taken out by tapping upon the back with a nail set, using a small piece of polished steel to lay the face of the plate on, and localizing the spot with a pair of calipers. The part raised by the tapping, cut away with the scraper, then rubthe spot with Scotch stone and water, then a piece of engraver's charcoal (cut to a pencil point), with machine oil; then burnish with the regular engraver's burnisher and sperm oil, finishing with rouge and refined turpentine.
When the plate is well polished, make a strong solution of caustic potash (C.P.), which comes in sticks, as strong as possible, as long as it does not stain the copper. It should register about 40 deg. with an actinometer used to test silver solutions.
Take a piece of absorbent cotton and clean the copper with potash (by the way, use finger tips); rinse under tap for five minutes, then a fresh piece of cotton with alcohol at 95 per cent., rinse again with water, and place in warm water for final rinsing; stand up on corner, or place in drying frame usually used for negatives; allow to drain. Should any stains appear, it must be recleaned and all the operations repeated until it drains off without streaks, for these streaks and spots of stain are caused by the caustic potassa, which is difficult to remove. It is as hard to get rid of from the copper as hyposulphite is from a negative. These streaks retard the acid on the copper wherever they appear, and cause defects in the recording of the original tones of the negative.
The plate is then ready for graining.
A grain is required on the copper plate so that the tones will be reproduced, as copper has not a sufficient grain of its own. The grain is given to the copper plate by dusting it with powdered Syrian asphaltum or resin. Have a paste-board box made 18 inches high, 12 inches wide and 8 inches deep, perfectly air-tight, with a small door running the whole length on the widest side, an inch or two from the bottom. Have the inside of the box perfectly smooth; place withinthe box 4 ounces finely powdered Syrian asphaltum (sold by Messrs. Theodore Metcalf & Co., Tremont Street, Boston, Mass.); it is difficult to find in New York. Shake the box vigorously, place on table, insert a piece of wood an inch high made in shape of cross (or open square, or have netting of wire raised an inch from the bottom of the box); the copper plate, previously cleaned, is at once placed face up upon it. Instead of shaking the box it can be arranged upon supports (see fig. 1), and revolved.
Fig. 1.Fig. 1.
Close the door instantly, and let the plate remain about two minutes; carefully remove the plate and place it on a Florence oil lamp, holding the plate with a hand vise, watch carefully until the powder disappears from the surface and the plate slightly smokes, then stand aside to cool. Do not keep the plate too long on the heater, or the particles of dust will run together, forming an impenetrable varnish over the plate. This part of the process is not difficult, but requires practice. Preserve each atom of dust as much as possible, examine with magnifying glass and, when cool, test with finger nail; if it rubs off easily, it has not been heated enough; then the plate must be re-cleaned and again powdered. To get a good all-round working grain, suitable for medium subjects, the plate should be placed at once in the box after shaking; thus the coarser particles that fall first, and the finer, which gradually settle, will combine after two or three minutes.
Many combinations will be suggested to the student by practice to suit the subject; for instance, waiting for two minutes and then inserting the plate, gives a fine grain for delicate subjects. Powdered dragon's blood (resin) in combination with asphaltum makes a beautiful grain; a separate box may be used for the dragon's blood; the asphaltum first dusted on the plate, then inserted in the dragon's blood box for twenty or thirty seconds, then melted together. The dragon's blood melts first, then the asphaltum.
The air brush is also used by professionals; it throws a resinous spirit varnish, coarse or fine, as required.
Have a wooden box made 24 inches long, by 12 inches high, 12 inches wide, with door 6 inches high on side, fastened with hinges, top and bottom of box open; cover top with sheet zinc. Place inside Florence oil lamp; the door is valuable to regulate the heat. On top of box place deep porcelain tray, 11 × 14, fill with water half full; in the water place two pieces of plate glass ½ inch high and 4 inches long, on which to place copper to keep it from the bottom of the tray. Slide the copper plate into the water, removing all air bubbles, keeping the fingers off the surface of the plate. Take the sensitized and exposed tissue and place face down in the cold water (65 deg.) sliding it gradually in at further end of the paper so as to avoid air bubbles; the instant the paper curls backward, place it over the copper plate and remove it quickly from the water. This has to be done with celerity or it will be found difficult to mount the tissue with the squeegee on the copper, and also it should be exactly placed with reference to the top and sides of the copper; all this, of course, to be done under water, never allow it to slip up out of it. Place plate on table and squeegee into place, stroking firmly from the centre, each way. Place face down on clean blotting paper, under heavy weight for fifteen minutes. While plate is under pressure (which is necessary to enable the gelatine to expand and attach itself to the plate), start the lamp gradually, and by the time the paper is ready the water should register on the thermometer 75 deg.Slide the plate under water removing air bubbles as they appear, with a ball of absorbent cotton; when the heat of the water reaches 90 deg. Fahr., the gelatine commences to ooze from all around the edges of the paper, and after the plate has remained in the water about ten minutes after the showing up of the gelatine (at the temperature from 90 to 95 deg.), take a pin and carefully raise the paper at the corner, gradually pulling away the paper toward the opposite corner, keeping the hand close to the water; should the gelatine which adheres to the plate appear to lift, wait a few minutes longer and start another corner. After the paper is stripped from the plate, gently develop the negative resist with a piece of fresh absorbent cotton, delicately rubbing the surface, edges first, and lave the plate up and down in the water, keeping the temperature steadily at 90 to 95 deg., by raising or lowering the lamp. (Should the paper be under-exposed and appear very black on the copper, develop at 100 to 110 deg., not over. If over-exposed it will appear very thin, and the heat of the water must not go over 90 deg.; it will strip at 88 to 90 deg.) Then the negative image gradually appears, the darks first, which are of course the brightest portions; when all detail appears in the shadows and the negative stands out clear and bright, take it out of the dark-room (which is lighted with an ordinary lamp), and gently wash under the tap with clean and cold water at 65 to 70 deg.
Dry the resist with alcohol, pouring it over the plate from one end, starting with half alcohol and half water, gradually adding more alcohol and eliminating the water, until a final flooding with absolute alcohol is reached; use fresh solutions of alcohol and water for each copper; don't use old alcohol for anything except cleaning the copper at the end, and for removing the spirit varnish. Stand up to dry against the wall, face out, and standing square on the bottom of theplate, in the same position as you flooded it with alcohol; it will be dry in twenty minutes if rightly flooded. The bare copper should now be protected by a strong varnish in alcohol (it must flow freely off the brush); a good varnish for this purpose, and the best I know of, is an etcher's asphaltum stopping-out varnish, sold by Messrs. Devoe & Co., New York; price 50 cents per bottle. Should it get thick as you come to the bottom of the bottle, add a little spirits of lavender until it flows again freely. Take an architect's ruling pen and carefully rule a line with the varnish up to the edge of the picture, making it exactly true with the sides of the plate and the space on each side of the work the same with the top, the bottom space slightly larger; make sure that it slightly comes inside the picture. Keep the rule away from the surface of the gelatine, as it is very delicate. Then cover all the rest of the copper, protecting the bare parts and bevel, and bringing the varnish up to the line. Allow to dry hard; about twenty minutes will do. Form a wall about the resist, with walling wax, about an inch high; make a lip at one corner, the further left hand one, for instance; see that there are no leaks. There are several grades of wax, but Liedel & Co.'s is the best; when ordering you should give the name as modeling wax; gray is a good color. Pans can be used made of tin and varnished, or porcelain trays, protecting the back and edges of the plate with varnish, but I find the wax very helpful, especially on large plates.
Perchloride of iron C.P. is the acid generally used for this purpose; it is a still acid, and if the room is well ventilated no harm to health results, but care must be taken to air the baths after making to get rid of the surplus chlorine.
Four baths are used, each of different strengths, the strongest is used first, the weakest last. I quote from the catalogue of the Boston Art Museum, of the exhibition illustrating the technical methods of the Reproductive Arts and Photo-Mechanical Processes, held January 8, 1892: "Photo-aquatint (photogravure) for the production of half-tone intaglio plates from photographs from nature, paintings, etc. A dry aquatint ground is laid on a metal plate, and over this is mounted a gelatine negative film, made by the pigment printing process. To obtain this negative film areversedpositive on glass has first to be made. The reason why this positive must be reversed will become clear when the nature of the manipulations in the pigment printing process, which involves the turning of the film, are considered. The film mounted on the plate is a washout relief, thickest in those parts which are to show white in the impressions from the plate, and gradually growing thinner toward the darkest parts, where it is thinnest. The film acts as a 'resist' to the mordant, allowing it to pass freely in the thinnest parts, and less freely as it increases in thickness. If, however, the film were mounted on the bare plate, and the biting then proceeded with, the result would be of no practical use, as the plate would present merely shallow hollows, incapable ofholding the ink, and which would therefore be wiped out in the attempt to clean the surface of the plate. This is, however, prevented by the aquatint ground, which allows the mordant to circulate only in the channels around the resinous particles of which it consists, and thus produces a grain precisely as in ordinary aquatinting. The mordant used in perchloride of iron, which is a 'still mordant,'i.e., one which does not evolve bubbles of gas. An effervescent mordant cannot be used as the bubbles rising under the film would tear it up. In biting, successive baths of varying strength are made.
"A strong solution of perchloride of iron penetrates only the thinner parts of the film, whereas a weaker acts also through the thicker parts. The biting, therefore, begins with a strong solution, which acts only in the darkest parts, and followed up with weaker and weaker solutions, which continue the biting in the darks and at the same time carry it on gradually toward the lights. If necessary, the plate is worked over with the burnisher to brighten the lights, and with roulettes, etc., to strengthen the darks."
Purchase nine (9) pounds of perchloride of iron in crystals (45 cents per pound), take a wide-mouthed gallon jar, place within half a gallon of distilled water, add the iron until it tests 30 deg. by a Beaumé hydrometer, pour off enough to fill a one-litre glass stoppered bottle, after filtering through absorbent cotton. Keep adding the iron to the jar until the strength of each bath is reached. To the strongest solution add half a drachm of C. P. muriatic acid, and to the weakest half a drachm C. P. nitric acid; the nitric acid is added so that in the last biting a good final nip is given to the copper.
I here give my own formula, with those recommended by others.
The four (4) baths should be well aired for a day (in broad pans) in the open air before filtering.
(H. R. Blaney.)
No. 1shouldregistertoBeaumé'sscale42 deg.No. 2"""""37 deg.No. 3"""""33 deg.No. 4"""""30 deg.
The temperature of the bath to be at 63 deg. Fahr. when tested.
(Denison's.)
No. 1 should be made to register Beaumé's scale, 45 deg., the percentage of perchloride in this solution is 47, and the specific gravity 1.444.
No. 2, 40deg.;percentage,41;spec. grav.1.375No. 3, 38""38;"1.339No. 4, 35""35;"1.313No. 5, 27""27;"1.225
From an article in thePhotographic News(English), Nov. 1, 1889, as practiced in India.
(Waterhouse.)
No. 1,sp. grav.,1.444;ap. per ct. ofFe2, Cl6=47No. 2,"1.375;""41No. 3,"1.339;""38No. 4,"1.313;""35No. 5,"1.225;""27
A stronger solution of 48 deg. has been tried (by the above) but has no penetrating power through even the thinnest film.
For large plates, 20 lbs. perchloride of iron and distilled water, until weight amounts to 1.500 grammes per 1000 c.c. From this four (4) solutions are made, at
No. 1, 42deg.Beaumé;spec. grav.1.420No. 2, 38"""1.375No. 3, 35"""1.330No. 4, 31"""1.285
The plate is now ready for biting. Keep a record of the bitings, and length of time for each one, for after-study; also note the time of exposure of the tissue, age of same, etc., etc.
Pour the acid from a glass graduate with one sweep over the plate, removing all bubbles with a feather, noting the time of immersion so as to guide you. Start with 42 deg., having ready the 37 deg. in another graduate, watch carefully the action of the acid, and if the resist has been properly printed, the action of the acid will show after a minute; if longer it means a generally longer biting for each bath.
42 deg., No. 15 minutes37 deg., No. 25 minutes33 deg., No. 32 minutes30 deg., No. 42 minutesTemperature of bath at 70 deg. Fahr., with No. 103 tissue.
Total of different bitings, from 10 to 25 minutes, according to depth of printing. It always varies. There is no hard and fast rule; you must in time learn to judge by your eye alone. The acid will first attack the thinnest part of the film, wherever that may be, and when the darkening of the copper ceases to spread to the next thickest parts, instantly pour off the acid, and pour on the 37 deg. Do not allow the atmosphere to act on the gelatine while biting any longer than is necessary to pour off one bath and quickly pour on a new one. The 37 and 33 deg. baths are for the middle tones, the 30 deg. for the most delicate ones. The action of each bath is cumulative, the 37 deg. biting a little where the 42 deg. had bitten, the 33 deg. doing the same for those before it, besides taking care of itself, and the 30 deg. attacking all more or less. During the biting with the 30 deg. solution, it should be continued until the whites just turn color, and a minute beyond; that is, the copper should begin to show a very little under the thickest and darkest film.
(Note that in the carbon resist the shadows are transparent and the high lights are opaque.)
The length of the last biting very seldom is over two minutes. It is better to overbite your darks, and underbite your lights, if you vary any.
The amount of moisture in the air and the heat of the day influences the length of biting. In hot weather in summer it is very difficult to work the process, the walling wax being discarded and the copper (back and edges protected by varnish) placed in a porcelain tray, surrounded by ice-water and kept at 65 to 70 deg., and the acid pured over the plate to the depth of one inch.