CHAPTER V.

Chemicals and Materials for Collotype.

GELATINE.—The basis of the Collotype process is gelatine—the finest purified animal glue—and the important part it plays in the process demands an acquaintance with its properties, which are found to differ widely, according to its origin and method of preparation. Gelatine suitable for Collotype should be free from all impurities—particularly alum, which is not infrequently added in small quantities during manufacture. Dr. Eder, in particular, has made most minute examinations of various samples, more especially with regard to their suitability for Collotype, Carbon, and the Galvanographic processes, and his opinion, agreeing with that of other experimentalists, is here quoted:—

The two principal modifications of gelatine are those produced from bones, sinews and skins of animals, and from the air bladder of the sturgeon, the latter variety, better known under the name of isinglass, represents, from a chemical point of view, glue in the purest form. On account of its high price and ever-varying quality as found in commerce, it must in practice give place to a glue or size containing chondrin, and prepared from bone and gristle. Another objection is, that plates prepared with an admixture of isinglass are less durable than those containing gelatine only. On the other hand, it certainly possesses advantages of its own, which will be more fully considered hereafter. A gelatine suitable for collotype must absorb plenty of water and swell out, but not liquefy after having been soaked in cold water for twenty-four hours. Gelatine or isinglass prepared from the bladder will not stand this test, but, on the contrary, partially disintegrate. The consistency and bearing capacity of a jelly formed by dissolving gelatine in water by means of heat and allowing it to cool, form, in the opinion of Lipowitz, a safe and reliable means of forming a judgment as to its suitability for collotype. Dissolve by first soaking five grammes in 45 cm. of distilled water for several hours, afterwards apply heat, and allow to stand twelve hours to set. For purposes of comparison this should always be done in a glass of the same diameter. A small tin or zinc slip is placed across the top of the glass vessel containing the jelly; through the centre of the tin a hole is pierced, through which a wire moves easily up and down. To its bottom end is soldered a piece of tin, of about 15 cm. diameter, the convex side lying upon the surface of the gelatine. On the other or upper end a small funnel is placed, large enough to contain 1-1/2 kilo. of small shot. The weight of the wire and funnel being ascertained, it will be a simple matter, by also weighing the shot, to calculate the weight required to sink the instrument into the jelly, and to ascertain with accuracy its weight-bearing capacity and proof of the relative value of various samples for Collotype printing. If otherwise suitable, the variety capable of supporting the greatest weight before yielding will be the one to select. The power of absorption of different samples varies considerably; for example, the Collotype gelatine of Creutz absorbs in two hours 30 parts of cold water poured over it. Nelson’s patent opaque, in the meantime, only absorbs five parts, but after a further period of 4-1/2 hours the difference is equalised, as it will absorb 25 parts more of water, making altogether exactly 30 parts. This experiment proves the latter to be a harder quality than that of Creutz, and it necessarily requires a somewhat different treatment for the production of the best results.

Isinglass or fish glue, although, as before stated, a finer quality of glutin than the ordinary gelatine of commerce, is, on account of its high price and varying quality, comparatively little used. The addition of this substance also softens the gelatine layer. On the other hand, it possesses certain properties which render its use at times desirable; for instance, it does not dry so completely as gelatine, the layer prepared with it consequently gives cleaner and more even proofs, a point particularly noticeable in the earlier impressions. Such prints will also exhibit greater brilliancy, and this is of especial consequence in work produced from photographs from nature, in which class of work the half-tones are too often wanting. Some operators take even equal parts of isinglass and gelatine, but such layers must be very thin, and dried quickly. Thick layers are liable to crack or fly, and in use to refuse to take the ink, which must for such film be used in greater quantity and thinner than when gelatine alone is employed. A solution of isinglass is not so readily prepared as one of gelatine. It is necessary to allow it to soak in cold water for from twelve to twenty hours, and the temperature then gradually raised to nearly boiling point, at which heat it should be maintained for some time longer, and then filtered through flannel into the gelatine solution. In printing from these plates, particularly when the proportion of isinglass exceeds one-third the amount of gelatine, there is a disagreeable tendency of the printing paper to adhere in the high lights—that is, in those parts of the print layer which are represented in the negative by entire opacity—as in the sky of a landscape or a background which has been “stopped out” on the negative. On other portions of the plate where the light has more fully acted—during the printing in the printing frame—this troublesome defect will not be observed. The fault is more noticeable when the dull enamel paper is used, while a hard, well-sized letter paper presents the other extreme. Further details of printing papers will be found in another chapter.

In the introductory chapter of this work the photo-chemical qualifications of the bichromates of ammonium and potassium with regard to collotype are considered.

Bichromate of Potassium is found in the well-known rich orange-red crystals. It is readily soluble in cold water (1.10). It may be easily recrystallised and purified readily out of either a hot, warm, or cold solution, and forms in thin layers of fine crystals. Therefore, in using it for collotype, care is needed not to exceed the strictly necessary proportion, as the plate may be hopelessly spoiled by the partial crystallisation of the salt, which will probably show during the process of drying. The maximum addition of bichromate of potassium in proportion to the weight of the gelatine may differ according to the peculiarities of the latter. Dr. Eder considers about 3/10 to 4/10 by weight of this salt in a dry state may be added to a gelatine, which will absorb from six to ten times its weight of water, without fear of crystallisation. There are, however, many gelatines that are incapable of working with more than 2/10. Rapid drying of the plate in a current of air, and at a temperature of 40° C., will, to some extent, obviate this objectionable feature. By reason of its comparative insensitiveness to light and this facility of crystallisation, and notwithstanding its comparative low price, it is now far less used in collotype than

Bichromate of ammonium, the crystals of which somewhat resemble those of the potassium salt. In colour they are of a browner hue, and readily distinguishable, since when heated on platinum deflagrates, emitting sparks and leaving a residuum of very finely-divided oxide of chromium. The ammonium salt does not, however, so readily crystallise on account of its considerably greater solubility in water compared with bichromate of potassium. By comparison, 12 parts of the ammonium salt are equal to 15 parts of that of potassium. At least 1/3 to 1/4 more of the former may be used without disadvantage or fear of it crystallising out; it will, on the contrary, improve the quality of the film or layer. The ammonium salt will also be found comparatively more soluble in hot water, but care must be exercised that the solution never reaches the boiling point, neither should the heat of the drying box ever exceed 50° to 60° C., or the films will present a pale brown appearance, and will prove alike insensitive and partially insoluble. The single and tri-salts, offering no advantages over those already described, may be dismissed without further consideration. It may be remarked for non-chemists that the addition of any acid to the chromated gelatine solutions inclines to the formation of trichromates of the alkalis; and, on the contrary, the addition of free ammonia, or any other free alkali, produces simple chromates. The addition of ammonia to the chromated solution has been strongly advocated in many quarters, but as the result of numerous experiments, it may be definitely stated that the supposed advantages are more than counter-balanced by the numerous disadvantages arising from such addition. The mixing, in equal or other proportions, of the potassium and ammonium salts has been recommended, but there is no real gain or sufficient reason for preferring any of these combinations to the ammonium salt alone. Besides the chromates of the alkalis, several other salts as well as acids are used in Collotype printing, although seldom, and then only in small quantities and for special purposes, such, for instance, as hardening the print layer, or for the purpose of facilitating the retention of moisture in the film. For the former purpose small quantities of chrome alum are sometimes added; while for the latter chloride of sodium or chloride of calcium are sometimes added to the chromated gelatine solution at the time of coating the plates, or are added as ingredients to the so-called etching or damping solution which is frequently applied to the plates during the printing. Glycerine answers for this purpose, and is to be preferred. It should be borne in mind that all additions to the film to facilitate its retention of moisture lessen its adhesiveness to its support. And with regard to the direct addition of the substances mentioned to the film during the preparation of the plates, it is not very apparent how such readily-soluble matters can remain in the film after the prolonged and careful washing the plates should be subjected to after exposure in the printing frame. On these grounds the addition of such substances to the damping solution is more rational and certainly more effective.

Alum and Chrome-Alum causes the gelatine to set sooner without precipitation, and harden the layer considerably after drying, and although gelatine, as well as glue, always contains more or less chondrin, which is precipitated by alum, great care is necessary in the addition of the latter; and it is better that such addition should be made immediately before the hot solution be poured upon the plates, otherwise a precipitation will ensue, which, although very finely divided and at first scarcely visible, will become more apparent during the drying of the plates, and finally show in the picture in innumerable white spots.

Silicate of Soda is almost indispensable as a substratum in the preparation of collotype plates, although many practical workers still adhere to the method originally proposed and practised by J. Albert, of Munich, who used as a substratum a chromated solution of gelatine and albumen, rendered either partially or wholly insoluble by exposure to light. The silicate is here considered as having about the consistency of syrup. If more fluid the adhesive power of the substratum will not realise expectations, and the plates will require very careful handling if even a few hundred impressions are required.

The composition of the film, the exposure of the plate through the back, and the careful treatment of the layer in the actual printing operations are all matters which considerably affect the “life” of the plate, and to which we shall again return.

Chloride of Zinc is used for hardening special parts of the collotype plate, and is easily prepared by dissolving—to saturation—sheet zinc in hydrochloric acid, filtering, and reducing as circumstances may demand.

Tannin and Tannic Acid in strong solution harden the film, and may be applied to certain parts to cause it to “take” the ink. For instance, to remove, or rather harden, spots which refuse the ink and consequently show upon a dark ground; and also to write names or titles to the plates. Written additions of this character must, of course, be reversed, as in lithography. The addition of tannin to the chromated gelatine before coating the plate is recommended by Waterhouse, he considering such addition confers greater lasting power to the films so treated.

Cyanide of Potassium and Ammonia.—All alkalis or carbonates of the alkalis have a directly opposite effect upon the printed layer to that last referred to. By the judicious use of one or the other of these agents spots printing light or dark may be either removed or considerably modified.

Glycerine and Sugar and similar easily soluble organic substances, added to the chromated gelatine, would assist the easier and evener damping of the plate, but render it more susceptible to damage, and the desired effect is better attained by suitable additions to the damping water.

Organic Acids.—Citric acid and tartaric acid are recommended as additions to the chromated gelatine, partially as influencing the so-called “grain,” and also as additions to the damping solution to facilitate the production of cleaner prints. Experience with these substances shows that the print layer suffers from their use, and for clearing up a plate inclined to yield dirty impressions a weak solution of ammonia is far preferable.

Nitric Acid has even been recommended for occasionally damping the plate in certain cases, after which the most delicate half-tones should be reproducible. An experienced collotypist ought never to be under the necessity of adopting such desperate remedies, which more often than not rapidly and entirely spoil the plate, and it is better to at once prepare a new one.

Ox Gall remains to be mentioned as exercising a beneficial influence upon the print layer under certain circumstances. It considerably hardens the gelatine, and tends to reduce the annoying tendency of the film to adhere to the paper in the high lights. Ox gall was first used at the Royal Printing Works in the “glass” printing process there utilised for the reproduction of line subjects. Some operators of eminence consider that in collotype its use should be strictly confined to lineal representations, as by its continued use the half-tones are partially lost.

Ink and Paper.

Of great importance is the careful selection of the materials for the printing proper—i.e., the printing inks and papers, and the sometimes necessary admixtures of the former.

Printing Ink.—The most simple and suitable of all greasy or oily printing ink is the best chalk ink, as used in lithography. It is composed of finest lamp black thoroughly incorporated with boiled linseed oil (varnish). The collotypist must, in addition to different coloured ink for mixing with the black, be provided with thin boiled oil (middle varnish), and also turpentine and olive oil, the latter being used to thin or reduce the printing inks, which, as usually supplied to the trade, are too stiff for immediate use. When in printing it is necessary or intended to use an admixture of several colours, they must be first well ground in turpentine and the latter allowed to evaporate. A small addition to the chalk ink of indigo or Prussian blue greatly improves its colour and printing power, but may be considered more suitable for the reproduction of line or stipple subjects. A dash of Venetian red or carmine imparts to the chalk ink a warmer tone, resembling the colour of an ordinary silver print. Whoever ventures on colour printing must be able to prepare all the necessary combinations with the greatest exactness, and should use for each a special inking slab and roller. In the chapter dealing with the printing, further particulars will be given as to the mixing and thinning of the colours.

The Quality of Paper suitable for collotype may be next considered. Without doubt the best is that known as dull enamel, or chalk paper, which is now supplied of a pink tone as well as of the ordinary white quality. It should not be glossy or adhere too strongly to the printing surface, as the enamel will, under such circumstances, be partially or wholly torn from the paper, a difficulty equally observable whether the paper be used damp or dry. As previously observed, the adhesive power of the printing surface increases with the softness of the same. This may arise from using a small percentage of bichromate or a short exposure to light, in which cases it is always more apparent in such parts, as the densest portion of the negative may have been entirely protected during the copying in the printing frame, or it may arise from the addition of too great a proportion of isinglass. On the other hand, a gelatine film strongly chromated, proportionately longer exposed, and in which there is no isinglass, will be less likely to exhibit this annoying tendency.

Well-sized fine glazed papers generally adhere least, and are usually laid on dry. When the impressions are to be issued without glaze very thin fine-surfaced cardboard—not enamelled—is frequently used, and the margin of the printing plate covered with a suitable mask. Any well-sized paper possessing a fine surface may be used. Impressions on such paper may be passed through a weak solution of gelatine, and after drying, rolled, varnished, mounted and again rolled. Any retouching must be done immediately after the sizing, and before the application of the varnish. Enamelled papers require a special glaze, not a lacquer, or are finished without glaze at all.

CHAPTER VI.

Preparation of the Collotype Plate.

THE well-polished glass plate must be thoroughly cleaned with alcohol and ammonia, and it should be ascertained that the surface is entirely free from scratches. It is then coated with the substratum, consisting of a dilute solution of silicate of soda and albumen. The following formula is in constant use by many practical workers of experience, and answers every requirement:—

Distilled water8parts.
White of egg—whisked7”
Silicate of soda (of the consistency of syrup)3”

This mixture should be well beaten to a froth or put in a bottle with pieces of broken glass, well shaken, and placed aside to settle for half-a-day. Then filter through paper, which should be replaced so soon as the pores become choked. To facilitate the operation the filter paper should before use be moistened with distilled water. The surface of the plate should be flooded with water, and coated as evenly as possible with the above solution, a small glass rod being made use of to assist the flow. Air bubbles must be removed, and by quickly tilting the plate the superfluous solution run off; this should not be used to coat other plates without being again carefully filtered. If the substratum be thin, it is not necessary to wash the plate after drying, but after standing aside two days to harden, with dusting and warming, they may be coated with the chromated solution of gelatine. If small crystal formations appear on the surface the plate should be well washed and again dried before proceeding further. The following formula for substratum is due to M. Thiel, senior; it contains less silicate, and the washing before applying the sensitive coating is never necessary:—

Distilled water36parts.
White of egg20”
Silicate of soda6·2”

Stale Beer.—Albumen is sometimes replaced by beer, from which by frequent outpourings the carbonic acid has been removed, and to which caustic potash has been added until it fails to show an acid reaction when tested with blue litmus paper (about three to five grammes of caustic potash to the litre of beer will be necessary). To a litre of beer so prepared is added 1/3 kilo. of silicate of soda. The main points in this and also the following operations are careful filtration, even coating and drying in a clean place entirely free from dust. Plates previously used or somewhat scratched must before further use be thoroughly cleaned and the face reground with the finest emery powder.

The Sensitising Solution.—This is manifestly the most important matter in the Collotype process, for on it depends not only the durability of the printing plate, but also the method of procedure in the subsequent operations. It may be here intimated that plates prepared by the very best formula cannot be expected to yield good impressions if the whole of the manipulations are not carried out with the greatest care, and it should be noted

1. That neither too much nor too little of the chromated gelatine should be allowed to remain upon the plate. It is impossible to lay down a hard and fast rule, and the exact amount must be determined by the experience of the operator. Of equal importance, and naturally connected with this operation, is

2. The even distribution of the solution, the flow of which may be assisted or directed by the aid of a small glass rod, and by allowing it to run off more or less at each of the four corners of the plate. Considerable experience has demonstrated that the coating is applied most easily and in the evenest manner by resting the plate—previously warmed to about 45° C.—as horizontally as possible on the cork of a heavy bottle, the latter standing in the centre of an earthenware dish. A sufficient quantity of the solution, carefully filtered, and heated to 45° to 50° C., is then poured on to the centre of the plate and rapidly spread over it with a glass rod, previously well warmed. So soon as the surface of the plate has been entirely covered a further supply of the hot gelatine solution is quickly poured on and partially run off at each corner. A scarcely perceptible inclination should be given to the plate, otherwise the layer will be too thin; it ought to be about the thickness of four or five sheets of Rive paper of 8 kilo. per ream. It should be borne in mind that the addition of isinglass will necessitate a comparatively thinner layer than when gelatine is used alone.

To obviate the gelatine coating becoming uneven great care must be exercised in removing the plates—when coated—to the drying-box, the levelling screws of which have been adjusted with great accuracy. The box should be heated to a temperature not exceeding 50° C.

To satisfy the adherents of isinglass, two eminently satisfactory formulæ are here given—

With Gelatine only.
Collotype gelatine6grams.
Water700ccm.

The gelatine is first soaked in the water for one hour, and then complete solution effected at a moderate temperature, and the solution filtered through best filter paper. Boiling heat is never required. Japanese filter paper is the very best. Heat will be necessary during the operation, which may be conducted either in the drying box or a warm oven. When filtered, add, with constant stirring, 8 ccm. of a solution of bichromate of ammonium in water (1 in 8, so that 8 ccm. solution contains 1 gramme of the ammonium salt), which should also be thoroughly filtered before addition to the gelatine.

The combined solutions being well incorporated, clear and free from air bubbles, should, as previously described, be heated to 40° or 50° C., poured upon the warmed plate, and at once conveyed to the drying box, the temperature of which has been slightly raised. The box should be closed, and gradually increased to 50° C., at which heat it should be steadily maintained until the drying is complete, which usually occupies one to one and a-half hours, varying somewhat according to the thickness of the coating and the strength of air current, which is necessary to the proper and rapid drying of the films. It may, under some circumstances, be necessary to augment the admission of air; this may be done by slightly raising the lid of the drying box. The current must freely circulate in all parts of the box, and remain as constant as possible. If subjected to sudden fluctuations in the air current or temperature, the plates will dry unevenly, and endless troubles ensue.

The Grain.—The so-called grain which is necessary for the printing first shows itself on the completion of the washing of the plates after printing in the copying frame. It is a peculiar, net-like formation, varying in fineness, and similar in character to the unwelcome appearance known to carbon printers as reticulation. It arises in consequence of the uneven distribution of the soluble layer in comparison to the substratum. In Collotype, its production is promoted by thicker layers and slower drying at a temperature of 45° to 50° C. The addition of isinglass also seems to aid the production of grain. Certain varieties of gelatine which possess great power of absorption give a smaller grain. Creutz’s gelatine may be mentioned as an example of this class. An addition in small quantities of dilute acid, particularly those of organic origin, as acetic or tartaric, tend likewise to the production of grain. Insoluble powders, as of glass or flint stone, may be added, but only in the very finest state of division, and in very small quantities.

Formula with Isinglass. Two grammes of the best Russian isinglass are cut up into the smallest possible pieces with a pair of sharp scissors, soaked in 25 cc. of water for 12 hours, heated nearly to boiling point until solution is effected, and then filtered while hot through close flannel. Cover six grammes of good Collotype gelatine with 50 ccm. of water and allow to stand, heat nearly to boiling point and thoroughly incorporate the two solutions, again, while hot, filter through paper or close flannel. To this is added 11 cc. of the filtered solution (1·8) of bichromate of ammonium, as previously referred to. The complete solution is then heated to 50° C., or should it exceed that temperature it is allowed to cool to it, and the plates warmed, coated, and dried in the manner previously described. It is not advisable to overcrowd the drying-box by attempting to dry too many plates at the same time: it necessitates a too frequent examination of the contents and so interferes with the evenness of the drying, and as this is a point of even vital importance no risk should be incurred.

The plates being properly dried allow them to remain in the box for some time longer, the source of heat being either removed or extinguished, and the whole allowed to cool gradually. If the plates are removed too quickly in their fully-heated condition—particularly when thickly coated—the too rapid contraction of the glass will crack and destroy the film; by gradual cooling such accidents are avoided. When finished the plates may be kept in a cool, dry place for two or three days. Many operators keep them this length of time purposely, believing they thereby obtain finer results. The plates so kept must, of course, be thoroughly shielded from actinic light.

Negatives, with regard to their suitability for Collotype, may here be considered, as the after result naturally depends considerably on their adaptability to the process. They should be quite free from fog or yellow stain, and for half-tone subjects not too intense. The greatest difficulties will present themselves in the production of portraits or such work from nature by the Collotype process. Photographer and printer ought, so to speak, to work hand-in-hand. The negative must show the finest gradation from clear glass in the shadows to the necessary density in the high lights, quite free from veil or fog, and intensified very little, if at all. They should be produced in a studio where the light is thoroughly under control. If not to be stripped all negatives should be taken on patent plate and protected by a coat of varnish, sufficiently thin to offer no obstacle to the production of sharp prints, and yet sufficient to obviate in a great measure the risk of damage by the pressure it is subjected to in the pressure frame in contact with the unyielding surface of the Collotype plate. The risk of breakage at this stage always renders stripped films preferable, their elasticity better enabling them to withstand the pressure. The gelatine layer applied to the negative before stripping must be as thin and even as possible, as from thick or uneven films it is almost impossible to produce a sharp print. If the negative is of a line subject, and it is desirable to have clear whites that the impressions may resemble as much as possible the original, the lines in the negative must be clear, but the remaining portions of great density. The ink for such work should consist only of pure chalk printing ink, which will be generally found to give stronger contrasts than the usual mixed Collotype inks.

Retouching Negatives and Plates.—The negatives must have all defects made good by the retoucher, all blemishes being moderated as far as possible. Retouching on the Collotype plate or print should not be required; but this is seldom so, as during the printing the negatives frequently sustain slight damages. Retouching the printing surface is seldom very successful. The alteration easiest effected is the removal of small light spots which refuse to take the ink, and such may be removed by touching with a fine brush dipped in a strong aqueous solution of tannin, this alteration being made upon the finished plate after it has been washed and dried. With this tannin solution inscriptions may be inserted which have to appear black in the print. Or a solution of gum arabic, to which has been added a solution of bichromate of potassium, may be used, and after such addition the plate is once again exposed to light. Writing which has received too much exposure, and which should appear in the finished print black on a white or light ground, often appears white. This defect arises from the gelatine on which the light has had little or no action swelling to such an extent that the letters are buried or sunk into the plate too deep to be touched by the roller when the plate is inked. It may be rectified by further thinning the ink with varnish. The removal of dark spots from the print layer can never be completely effected. Ammonia, cyanide of potassium, oxalic and other acids have from time to time been recommended, but this treatment will prove effective for only a few impressions.

Negatives should therefore be very carefully and systematically retouched and carefully preserved from damage during the printing. Another method of applying writing, such as the names of firms, &c., is to get them printed from small type on thin gelatine films and attach them to a clear or erased portion of the negative during the printing in the pressure frame. Writing so added will appear as white letters on a black ground. It will often be found practicable to attach the written or printed matter to the original while photographing, and so include it in the negative at one operation.

Printing under the Negative.—The printing of half-tone plates is best performed in diffused light; to hit the exact exposure is as important as it is difficult, and depends not only on the actinic power of the prevailing light, but also on the quality of the negative and the sensitiveness of the layer. The greater the amount of bichromate the latter contains, the greater its sensitiveness, and consequently the exposure is shorter than for one containing less of the sensitising salt. A thinner layer must not be printed so long as a thicker one, or dark prints, lacking in contrast, will result. On the other hand, a thicker layer may be submitted to a comparatively longer exposure without harm; the exact time must not be greatly exceeded or it will happen that the blackest portions, written titles for instance, will not take the ink. During printing, one may with advantage shade such portions a little. The examination of the back of the Collotype plate is the safest method of judging the correct exposure during the copying, but every precaution must be observed during such examinations that neither the plate nor negative be moved from their original position. Only when the negative is very clear may the picture be seen by reflected light as a positive; it will in such cases appear on a dark ground. It will be found difficult, particularly for a beginner, when making an examination in the copying frame, to distinguish between the dark original picture and the pale brown image of the Collotype plate, as they naturally completely cover each other. By the dark brown tone of the uncovered margin of the print layer a certain indication is afforded of the completion of the copying.

An Actinometer may be used as an exposure gauge, as in carbon printing, and as a slight indication of the necessary exposure. When using the small cube[H] photometer with chloride of silver paper, five to seven tints may be printed, but the different modifying circumstances previously referred to must all be duly considered. Under a clouded sky in winter, using a dense negative, it may take a whole day to complete the copying, while in clear direct sunlight, perhaps, from the same negative, a quarter of an hour would suffice.

[H] The scale actinometer is one requiring less attention.—Trans.

Masking should be resorted to to form a margin at least an inch wide round the four sides of the negative, either by painting round the same with Indian ink, or by attaching to the negative strips of opaque paper, or, better still, thin tinfoil.

Sunning.—When sufficiently exposed, i.e., when the brown picture with all its half-tones and details is complete, the plate is removed from the copying frame and placed, face downwards, upon a piece of black velvet, and exposed for a short time—five to ten minutes—to a good but not direct sunlight, so that it may pass through the glass plate and harden the back of the print layer. A sheet of ground glass may be placed over the plate to thoroughly diffuse the light, and prevent any scratches or damages to the back of the glass plate producing any detrimental effect upon the printing surface.

Washing the Plate is the next operation. The plate is removed into the dark room and placed in clean cold water, which should be often changed—every half-hour—unless an automatic washing apparatus[I] is used; this is continued until the plate no longer has a yellow appearance, and the picture is visible only as a slight brown image.

[I] Jefferies’s Patent “Perfect” Washer is a very suitable appliance, and may be used for Collotype plates, negatives, or paper prints.—Trans.

Hardening the Film is effected by soaking it for a quarter of an hour in a one per cent. solution of alum. The plate is again washed, and the surface evenly dried with fine tissue paper, and placed aside for at least two days that it may thoroughly dry.

An Alcohol Bath is used by many operators to immerse the plate in for half an hour after drying it with paper, perhaps with the intention of drying the film quicker, or to strengthen the layer and assist the formation of the grain, but there is no gain by this procedure, and, as the bath is an expense, it may be omitted.

The Damping Solution, or so-called etching fluid, is of much greater importance, and it is necessary that the plate be submitted to it before printing be attempted.

Tinted Grounds often enhance the appearance of prints, and may be imparted to them by the addition of an aqueous solution of red, yellow, or other suitable colour, to the damping solution, which is applied in the ordinary manner. The plate is superficially dried, inked up, and printed from in the usual way.