The prints to be treated should be printed darker than for ordinary mounting, care being taken not to tone them very much, or a cold blue or grey will be the result when finished. The glass plate intended for the reception of the print should be free from scratches or blisters, and must be most carefully polished with rouge, tripoli, or the usual polishing mixture. It is then placed on a water oven or heated over a gas-flame or other contrivance, until it can be comfortably borne on the back of the hand. Hot gelatine solution - 1 part gelatine to 15 of water - should next be poured in a pool on the warm plate, and equally distributed over its surface by means of a glass rod; the prints, which are slightly smaller than the glass plate, are withdrawn from a weak solution of gelatine (1 part gelatine to 40 of water) and placed, albumenised side downwards, on the gelatinised glass plate, when it will be found that a good squeegeeing will remove the excess. When dry, the operation is complete. As a matter of fact, so largely have photographers availed themselves of this method of exhibiting their own productions, that it would be difficult to find a place of importance where the process is not employed.
And here we may mention that we have noticed, in a few instances where show-cases have been exposed to considerable variations of temperature, including the full force of the sun's rays, portions of the photographs have dragged away from the glass support, and in a short time showed unmistakable signs of fading, while the parts still adhering appeared comparatively fresh. There is always a danger of the print coming up in patches unless the glass plate is thoroughly well polished, and as the instances we speak of were all pictures of large size, doubtless the defect was due to imperfect polishing.
By abrading the back of the print with fine glass-paper, as recommended in the crystoleum process, it is possible, especially with landscapes, to produce a certain amount of coloured effects by means of liquid dyes in alcohol, and in many cases we have seen good results. When portraits, however, are so treated, they are seldom all that can be desired, the main difficulty being to get the proper depth of tint. The only difficulty likely to be encountered in the production of these pictures will be with the gelatine. Temperature, as in carbon printing, plays an important part: 65° F. (18° C.) will be found the most satisfactory temperature for the coating and drying room, and every precaution should be taken to obtain that result, or failures are sure to follow. Having regulated the temperature, the following articles will be required: - A flat tin dish for dissolving the gelatine, arranged over a Fletcher air-burner gas-stove, and a similar dish for soaking the prints in the melted and filtered gelatine; also a jug for pouring the solution over the glass plate, arranged over a fine jet of sufficient heat to prevent the solution cooling; a Wedgwood funnel, into the neck of which is placed a piece of clean wet sponge, answers well for filtering; a glass rod for distributing the gelatine over the plate; a rubber squeegee, and a piece of American cloth to protect the print while squeegeeing; strips of wood 1 in. broad, of any length, 2 such strips joined by cross-pieces forming racks for drying; padded blocks of the same size and shape as the medallions will be useful (the pad forms a cushion for the glass, while the raised surface allows of passing the burnishing tool quickly round when fastening down the rim); a stock of prints trimmed to the desired size and shape.
In the majority of cases, 1-2 in. margin of clear glass is allowed, this margin being afterwards filled in with Bates' dead-black varnish, as described below. A stock of oval dome-top or other shape glasses; suitable backs for these shapes fitted with rings; metal rims to fit; Bates' dead-black varnish; Young's patent size, as sold at the oilman's, completes the list.
Melting the gelatine: - Cover the bottom of the tin with size broken up into small pieces. Enough cold water is poured over to cover the pieces, when the temperature is raised to nearly 200° F. (93° C), to ensure perfect solution. When melted, a portion of the solution is passed through the sponge into the second dish for soaking the prints, and the remaining portion is passed into the jug, and kept hot by the means above stated. A well-polished plate is heated to 100° F. (38° C.) placed on a level slab, and covered with a pool of hot size. Should the solution prove refractory, the glass rod will assist spreading. Quickly transfer a soaked print from the second dish of hot size to its position, face down, on the plate, and roughly squeegee; remove the excess with a sponge, and again apply the squeegee, this time interposing a piece of American cloth to protect the print. If no air-bells are seen when examined from the front, the plate is placed on the rack to dry, paper side uppermost, which in a good current of dry air will occupy a couple of hours. When dry, the back should receive a second coating in like manner, another 2 hours being required before the next operation.
When the second coating is quite dry, a brush well charged with Bates' black is passed round the margin, completely coating the bare glass, care being exercised not to allow the varnish to over-
Up the photograph more than is really-necessary; 30 minutes' drying will remove the last traces of tackiness, and no more remains to be done than to fit the back, adjust the metal rim, and secure it down neatly with a burnishing tool to the padded block.
Landscapes and figure studies, nicely vignetted to the edge of the plate, are very effective when mounted as described above, on oval glass plates, in which case the use of black varnish is, of course, dispensed with. (Photo. News.)
At the works of the Ceramic Stained Glass and Vitrified Stained Glass Company at Chingford the dusting-on process is adopted. Sensitive plates are prepared, upon which the enamel colour is dusted; and it does not follow by any means that the glass or porcelain first coated with the sensitive mixture is that upon which the enamel is finally vitrified, as it is quite easy to transfer the dust image to a new glass or tile. If, however, the image is to be transferred, the glass should be collodionised and allowed to dry before the application of the sensitive mixture. The exact composition of the sensitive preparation used is a secret, but the mixture recommended for this purpose by Dr. Liesegang, which answers very well, is as follows: -
Water ...... 100 parts
Moist sugar .. .. 10 "
Gum arabic .. .. 10 "
Ammonium bichromate 4 "
The glass, very carefully cleaned (and collodionised if the image is to be transferred), is now placed on a levelling stand, flooded with the sensitive mixture, and after the composition has been allowed to remain on for a few seconds, the excess is drained off, the plate being now dried in an inclined position. The drying cupboard is contained in the "dark" - or rather, yellow-lighted - portion of the building; but no doors separate the dark room from the rest, the entry of white light being prevented by hangings of baize arranged on the baffle-plate principle, so that one can walk in or out of the yellow room without touching or disturbing the hangings. The warm cupboard stands on a tin water, vessel, scarcely 2 in. deep, and about 2 ft. wide by 5 ft. long, the whole being closed, excepting that a pipe is provided, by which any vapour may escape. The wooden bottom of the drying box stands directly upon the top of the hot-plate or water-bath, and the front of the box is merely closed by means of a curtain. As the plates are coated, they are reared up on edge in the cupboard, and allowed to remain until quite dry, when they are ready for exposure in the printing frame.
Much depends upon care in drying, and many fail in the working of the dusting-on process through drying the plates at too high a temperature; indeed warm weather often demands no artificial heat at all. To the hand, the interior of the drying box seems only a trifle warmer than the external air, and one may perhaps estimate it at about 85° F. (29 1/2° C). The large flat water-bath or hot-plate upon which the cupboard stands is kept sufficiently warm by one paraffin heater with a 4-in. wick, and turned down very low. The special advantage of the hot-water plate is that it ensures a uniform heat all through the bath; very little over-heating is fatal, as it bakes the mixture, and renders it incapable of again absorbing moisture.
The exposure is made under a transparent positive, and the greatest care is exercised to see that printing frames and transparencies are perfectly dry; indeed, it is generally considered advisable even to slightly warm them before use. An apparatus provided with a heating arrangement consisting of a battery of paraffin lamps, is where the printing frames are placed during damp weather; without some such arrangement for keeping them perfectly dry, the work would be uncertain. The exposure required is not a very prolonged one, a single minute in bright sunshine being often sufficient, while in dull daylight 1/2 hour or more may be required. No acti-nometer is used, it being easy for one who is constantly at the work to judge the exposures with sufficient accuracy.
The effect of exposure to light is to destroy the power of absorbing moisture which the sugar and gum ordinarily possess, consequently when the plate is withdrawn from the printing frame and exposed to damp air for a few minutes, those portions which have been protected from the action of the light will hold the verifiable pigment which is now dusted over the plate, while the most exposed parts refuse to take up any pigment, because they do not become adhesive by the absorption of water. For dusting with the vitriflable pigment - which is just such a powder colour as potters use in decorating their goods - the plate is laid in a tin dish, and the powder is dusted over with a broad camel-hair brush. If the image is very slow to appear, one may venture to breathe very cautiously on the plate, after which the enamel colour is again applied. A little consideration will show that in this process over-exposure results in a hard image, while fog or general tinting is a consequence of under-exposure. When a perfect picture, having all the gradations of the original, is obtained, the excess of pigment is brushed off, and the powder colour is fixed by flowing collodion over the plate.
If the plate were now fired, the chromium compounds in the film would become vitrified, and would give a disagreeable green tint to the picture, so some means of ensuring their removal must be adopted.
One secret is the composition of the fluid in which the plates are soaked at this stage to remove the chromium; but Dr. Liesegang recommends soaking for 1/2 hour or so in a 2 per cent. solution of caustic potash. The chromium having been removed, and the plate dried, all is ready for the final operation in the furnace, if the vitrification is to take place on the original support; but if the film is to be transferred, the plate must be allowed to remain in very dilute acid (say 1 part nitric acid in 60-80 of water) until the film can be floated off, and placed in position on the glass or tile which is to be decorated. (Photo. News,)