After six hours the first glass plate is detached from the layer of glue by loosening the latter around the edges with a knife blade, and the plate gradually lifted off commencing at one corner. With some care and skill, this operation is readily accomplished without detaching the gelatine mass.
5. Drying and detaching the veneers. The veneers with the gelatine layer still adhering to the glass plate are now dried. This is done in a heated room in which the veneers are arranged upon frames so that they stand almost perpendicular. The hot air for heating the room enters near the ceiling while the moist air is drawn away near the floor. The temperature of the lower zone where the fresh plates are placed should not exceed 68° F. The plates are moved up higher every day until, on the third or fourth day, they have become entirely dry. Before removing the veneers from the drying-room they should be tested in regard to their dryness. They are sufficiently dry, when, on pressing the finger nail upon the glue, no impression is made.
After removal from the drying-room the plates are allowed to cool off for at least two hours before detaching the veneers. The operation begins by detaching the gelatine layer on the edges with a very thin knife blade. The operator then takes hold of one corner of the veneer and draws it gradually and carefully from the glass plate. After trimming the edges the veneers are ready for use.
If the veneers are required to resist the action of water, mix with the solution of gelatine compounded with glycerine ⅓ fl. oz. of a solution of 5 parts of chrome-alum in 100 of water to every plate, and immerse the veneers for a short time after they have been detached from the first plate, in a similar solution of chrome-alum.
Veneers prepared by these methods can be used for various purposes in architecture and in the manufacture of furniture. Tennessee and other marbles have been so closely imitated, that when used for table plates, etc., the fact of their being imitations could only be detected by the closest scrutiny. The veneers are also much used for fancy and inlaid work, for coating columns, etc. To prevent their blistering and coming off, it is recommended to add one-quarter of its weight of glycerine to the glue with which they are to be attached to the articles.
Formo-Gelatine.
This product is used as a dressing in surgery, and, according to Samuel Rideal, is obtained as follows: Gelatine in aqueous solution is precipitated by formaldehyde H.COH, or CH2O, as a substance which on drying is a white powder, neutral, inodorous, and insoluble in water and dilute chemical agents. In commerce formaldehyde is met with in aqueous solution as “formalin” containing 40 per cent. of formaldehyde. If 1 Cc. of formalin be added to 200 Cc. of a 5 per cent. gelatine solution, the latter is changed into a gelatinous mass, not melted by heat nor reduced by water. If a smaller quantity of the formalin is added (1 in 1000; the jelly is said to be meltable, but with a higher tenacity) when dried it becomes insoluble in warm water. Formalin in less proportion, though it does not prevent the dried gelatine from dissolving in warm water, is said to improve the “body” of the jelly and its keeping qualities, and also the tenacity of the glue. The results of investigation show that with up to 1 per cent. of formalin the solidity of the jelly increases; above that it declines. Up to 0.02 per cent. (1 in 5,000) it redissolves in water after drying. Even with this small proportion the firmness of the jelly is sensibly increased. The English patent, 4,696, 1894, claims the addition of formalin during the manufacture of size and glue in such proportion that the product shall liquefy in warm water.
In examining commercial sheet gelatines for printers’ and photographic use, they were often found to contain small quantities of formalin. It seems to improve the quality, a very small percentage increasing the tenacity, flexibility and keeping qualities, while not affecting the transparency or rendering the material acid. When applied to articles that may be used as food, a strength of 1 in 50,000 in the final prepared product is not injurious to health, but should not be exceeded. (Rideal and Foulerton, Public Health, May, 1899, p. 568.)
Zimmermann applies dilute formaldehyde mixed with glycerine, vaseline, oil or yolks of eggs, with or without flour, to the surface of photographic films which are claimed to be thereby rendered more pliable and not so hard as when the formalin is applied by itself.
It will be seen that formaldehyde in more than traces renders gelatine insoluble; the product, moreover, is almost unaffected by water, is more or less hard and elastic and, owing to the antiseptic properties of the formaldehyde is nearly imperishable.