CHAPTER III.THE COLLOTYPE PROCESS.

This process, roughly described, consists in carefully grinding with fine emery, a plate of thick glass, then coating it with a mixture of white of egg, or of stale beer and silicate of soda. After the plate is dried it is rinsed in water, and dried again, then warmed, and coated with a thin film of bichromated gelatine. This coating is then dried in a suitable oven, after which the plate is exposed to light under a reversed negative, followed by a short exposure of the back of the film to light, then washed in cold water until the whole of the bichromate is removed from the film, when the plate is allowed to dry. It is next soaked in cold water, then in a mixture of glycerine and water, after which it is placed upon the press, and the printing proceeded with.

As in lithographic printing, the size of the paper upon which the picture is printed determines the size of the stone upon which the transfer is made, so as to give a good margin, not only large enough to prevent the paper being marked by the edge of the stone, but also to allow plenty of room for working the roller; so in collographic printing, the glass plate upon which the picture is to be made must be sufficiently large to allow a margin ofat leastfour inches all round the outside of the picture. This is not only requisite for the reasons that hold good in litho. printing, but for the additional one of giving plenty of room for the manipulation of the mask used to protect the margin of the print.

Collographic plates should also be sufficiently large to prevent their edges from cutting the composition rollers used for inking the image; for instance, a roller nine inches long should never be used upon a plate less than twelve inches wide.

Collographic films are made upon good patent plate-glass half an inch thick, free from scratches, with the edges nicely bevelled to avoid cutting the rollers; this bevelling the plates is absolutely necessary for use upon the machine, but is not so important for the hand press.

To prepare plates for use they are first ground with fine emery powder. To do this place one plate upon a smooth board or a couple of bearers across the sink, sprinkle with fine emery powder (sifted through book muslin), then with{164}water. Now, with another plate on the top, proceed to grind with a circular motion, continuing the grinding until the emery is worn smooth; then renew the supply of emery, sprinkle with water and again grind until the emery is smooth; now work well with water, squeegee dry, and examine the surface; if it is evenly ground the plates are ready for the next stage. If they are not ground all over, cover them again with emery and water, and continue to grind until the surfaces are of an even matt all over.

The plates being ground, they are well rinsed under the tap, and both sides are well rubbed with a rag to free them from all particles of grit, etc., then flood them with a mixture of

Liquor Ammonia1ounce.Alcohol1ounce.Water5ounces.

rub this well in with a clean rag, and again well wash with clean water, then allow to drain and flood with

THE PRELIMINARY COATING OFStale Beer5ounces.Water5ounces.Silicate of Soda (Waterglass)2ounces.

made up an hour or two before using, and filtered.

The wet plate is covered with this two or three times, and then placed on a rack to dry. When all the plates in the batch are coated the rack should be removed to the drying oven and the plates driedin sitû, not lying down.

Be as careful as possible to prevent any of the beer and silicate mixture from getting on the back of the plate, as it interferes with the running of the image afterward.

Plates may be prepared with the preliminary coating in quantity, as the coating hardens and improves by being kept.

When the coating on the plates is quite dry rinse them under the tap and again dry them, this time quite spontaneously. Lastly, they are placed upon the slab of the drying oven to get sufficiently warm before being coated with the sensitive mixture.

The sensitive mixture must be made exactly as follows, as everything depends upon the time and temperatures named: First of all, take thirty ounces of pure water and add ten drops of a saturated solution of chrome alum; shake up well and allow to stand ten minutes; if the water is very slightly opalescent, it is{165}quite satisfactory, but if there is any tinge of green then too much chrome alum has been used, and it must be thrown away and another lot made.

Then add two and one-half ounces of fine gelatine and allow it to soak until soft, then place it in the “Baine Maire” and melt the gelatine, stirring all the time. Raise the temperature to 125° F. and keep it thus for fifteen minutes; next add 150 grains of bichromate of potassium in fine powder, stirring until dissolved, and raise the temperature to 150° F. Strain the mixture through muslin into a porcelain jar and allow it to cool, keeping it in jelly at least twelve hours before using it.

This sensitive mixture does not work well if used to coat plates with directly it is made; therefore, it must be made up some time in advance. It keeps well, in fact improves, with keeping, up to about seven days. In remelting for use, only melt just sufficient for the plates to be coated, as it does not improve it to melt it too often.

Plates coated with the above mixture are dried at a temperature of from 100° to 120° F. in from two hours to three hours, and have a splendid grain.

Here is another formula the mixture from which does not keep, but must be made and used directly. It is the formula most frequently used by the writer. Plates prepared with it can be dried, at lower temperatures, in from twenty to fifty minutes. It is also the best formula when copper plates are used instead of glass, of which mention will be made at the end of the chapter.

Soak two and one-half ounces of gelatine in fifteen ounces of water until quite soft, then place all in the “Baine Maire” and melt at as low a temperature as possible; when melted add 100 grains of bichromate of potash and 50 grains of bichromate of ammonia both in fine powder, stirring until dissolved; now put in the thermometer, and raise the temperature to 140° F. and keep it there for ten minutes; then allow to cool to 125° F. and add the following mixture, stirring vigorously the while: fifteen ounces of alcohol and five ounces of a saturated solution of borax in alcohol. Mix the two before pouring them into the hot gelatine. With some gelatines the addition of this mixture will cause a slight coagulation of the gelatine, and it will stick to the stirring rod; therefore, the solution must be stirred until the coagulated gelatine is redissolved; then strain it through muslin into the pourer and at once use it by pouring it over the warm plates.

One of the best vessels to use as a pourer is an invalid’s cup with a spout at the side.

The above quantity of mixture is sufficient for sixteen plates 16 × 13 inches;{166}therefore, if only four are to be coated, which is the quantity that the drying oven to be described will take, the above quantities must be divided by four.

Now if the first formula be used, take sufficient of the jelly, melt it and strain it through muslin; then take one of the plates from the drying oven—which plate should be just warm enough to be comfortably held on the palm of the hand—carefully brush it over with a dusting-brush, so as to remove any particles of dust that may have adhered and balancing the plate on the palm of the hand, pour over it sufficient of the warm gelatine to just cover it. A 16 × 13 plate will take a little less than two ounces of the gelatine mixture. Take care not to spill any. Next place the plate upon the hot slab, and coat the rest in the same way. When all are done close the door of the oven and do not open it again until the plates are dry.

With the second formula themodus operandiis the same, except, of course, that instead of melting the already made jelly, the mixture made as above directed is used directly after mixing. The plates being dry they are ready for exposure, which need not necessarily take place at once, as if stored away in the dark and away from damp they will keep two or three weeks.

The exposure to light is made in a box-pattern frame, the negative having a mask in front of it to protect the margin of the plate from the light.

The negative used must be reversed—i. e., the image must be in its right position on the negative when viewed through the film, not as ordinary negatives are—through the glass; therefore, a mirror must be used in making the negative, or a skin negative may be used; or, if neither is practicable, the negative must be reproduced by one of the methods already given.

The exposure to light in the printing frame will vary according to the density of the negative, and the strength of the light, and should be continued until the image can be seen well defined upon the gelatine film when examined in the dark-room, by opening one-half of the back of the frame.

The exposure being complete, lay the printing frame face down upon a table in the light, remove the back and packing, and expose the back of the film to the light for a short time, without in any way disturbing the position of the collotype plate or the negative.

This exposure of the back of the image to the light has a twofold advantage, one being that it helps to weld the sensitized gelatine film to the surface of the plate, and so enables it to stand the wear and tear of printing; the other, that it reduces the relief of the picture, and so gives the inking roller a better chance of feeding the deep shadows.{167}

The duration of the exposure of the back of the film to the light will vary very much with the subject in hand; if it be one with slight contrasts, a much shorter time will be required than where the picture contains a lot of very heavy shadows.

The next step will be to remove the plate from the printing frame, and immerse it in clean cold water for five or six hours, or for such a time as suffices to remove the whole of the bichromate from the film. The plate is then placed in the rack to dry spontaneously, and should be allowed to stand for at least twelve hours afterward before printing from, so as to give the film a proper chance to become thoroughly hard.

We now have a collotype plate which has undergone the following operations, viz.:

Fig. 17.

A plate of thick glass ground with fine emery until of an even matt surface. Then it is washed and coated with the preliminary coating of beer and silicate of soda and dried. Then rinsed in cold water and again dried. Next placed on the warm slab of the drying oven until just warm enough to hold on the palm of hand. Then coated with the sensitive mixture, either No. 1 or No. 2, and placed in the oven until dried. Then exposed to the light under a reversed negative in a printing frame until the details of the image are well out. Then the back of the plate is exposed to the light for a short time. Then it is placed in clean cold water until the bichromate is removed. At last the plate is dried, after which it is ready for the printer.

Now, before proceeding further, it will be as well to touch upon various points of procedure, and also to describe what may be reckoned the most important piece of a collotype plant, viz., the drying oven. Figure 17 gives a drawing of the oven as it stands on the table for use, the{168}dimensions being from the top to the bottom of the frame 5 feet by 3 feet 4 inches wide by 3 feet deep. The top A, and the back, are covered with matched boards well seasoned; the sides and front, B, B, B, B, are made of frames 2 inches by 1 inch, covered with canvas sufficiently close to exclude light, but still open enough to let out the heat slowly. The door C is also a frame covered with canvas, 24 inches square, sliding upward in grooves, and balanced by means of weights. D is the jacket of iron upon which a copper tank rests, the jacket serving to carry the tank and to protect the wood-work from the gas flame used to warm the water in the tank. E is a sliding door in the jacket for the purpose of lighting the gas burner. F F are the projecting ends of a levelling arrangement forming the base of the oven, these ends being fitted with good, strong screws working upon iron plates let into the top of a strong table upon which the oven is placed. The distance between the two lower rails or styles is 6 inches, which is also the height of the iron jacket. D G is a tube let into the jacket to carry off the products of combustion from the gas. H is a tube with a screw nozzle, for filling the tank.

Fig. 18.Fig. 19.Fig. 20.

Figure 18 is the base of the oven,A A Abeing the three points where the levelling screws are placed; this base is formed of good 4 by 3 inch quartering, the other dimensions being as marked. This base stands on the table, the oven on the top, without any fastenings.{169}

Figure 19, the jacket of sheet iron well wired, with a tube at one corner, for carrying off the products of the combustion of the gas, and a hole at the other corner to carry the projecting tube from the tank, by which it is filled.

Figure 20, the copper tank, the outside dimensions of which are 37 by 31 inches, the flange fitting on top of the jacket; the depth of the tank in the centre is 3 inches; the inlet pipe is 1 inch bore, and 6 inches long, the bend being taken high enough to be about 2 inches above the top of the tank.

The base is placed upon the table, then the iron jacket on the top of the base; then the copper tank is fitted into the top of the jacket; upon the top of the tank is placed a slab of marble or slate 371⁄2by 311⁄2inches, cemented by means of plaster-of-Paris. The wooden top of the oven is fitted into position, the canvas being nailed on last. But, before doing this, it will be as well to fix the bearers as at C C C, Fig. 21, with corresponding ones on the other side, so that slats may be placed across for the purpose of holding heliotype plates, and for drying transfer paper, etc. The interstices between the edges of the slab and woodwork should be made good, either by means of wooden laths or cement.

Fig. 21.

The oven being erected, the first thing to do will be to fill the copper tank with water; then level the slab by means of the screws, or if the bottom ends of the base are shod with sheet iron, wedges may be used to level with. The drying oven must be kept in a room at a uniform temperature of about 60° F.

In using the oven after the plates are dry (after rinsing), place a piece of clean, dry, brown paper upon the slab, then place on this paper four plates 16 by 13 inches—which are the size and quantity for which the dimensions of the oven have been calculated; then place a thermometer between the plates, and close the oven. After the water has been brought to boil, the plates will be ready for coating; but, if the thermometer registers over 120°, turn off the gas, coat the plates, again close the door, and let the plates dry.

One of the first things the student must do is to familiarize himself with the oven. Note the time taken to bring the water to boil; the temperature of the{170}slab, and the time taken for the temperature to fall after turning off the gas. A day spent at this will teach far more than it is possible to do by writing.

Do not try to improve the oven by having the door to open any other way than by sliding. If made to open in or out, a draught of air is created which will cause mischief. Petroleum, instead of gas, may be used for heating the water.

Instead of using glass plates for carrying the collotype film, copper or brass plates may be used. They should be grained with fine sifted graining sand and a stone muller, then washed with plenty of fresh water, and coated with

Stale Beer5ounces.Water5ounces.Silicate of Soda (syrup)2ounces.Tannin1grain.

Prepare the mixture three or four hours before using, and filter it carefully. The subsequent operations are the same as with glass, except that the sunning of the back of the film must be dispensed with, and the second formula for the bichromated gelatine mixture must be used, as the first one gives too high a relief for copper.

Plates that have been used are cleaned again for further trial, by immersing them in a pickle of American potash or concentrated lye, then well washing, regrinding, washing, and finally coating with the beer and water-glass substratum, exactly as before mentioned.

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