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| Use of Dewaxed Bleached Shellac in Photographic and Engraving Industry.
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Adhesive Mounts
Adhesive mounting paper is much used in the photographic and printing industries to mount photographs and reproductions on a firm backing, such as paste board, by the mere application of heat.
Fine tissue paper is fed through an alcoholic solution of Dewaxed Bleached Shellac and rosin and then passed up through a drying tower to remove the alcohol. A sheet of this impregnated paper is then placed between the photograph and the mount and heat applied with a hot iron or press. |
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Photo-Engraving
This is a process whereby the surface of a metal is etched 'photographically' so as to prepare a printer's block suitable for the reproduction of pictures and diagrafts. The cold top enamel.
Process depends on the fact that a film of dewaxed bleached shellac from amonical.
Solution containing bichromates rapidly becomes insoluble on exposure to light, and hence a photographic image can be created on a metal place by dissolving away the unpolymerized portions and etching the exposed metal into relief by an acid.
Cold Top Enamel
60 parts pure Dewaxed Bleached Shellac are warmed with 75
parts ammonia (d,0.91) and 150 parts of water until dissolved,
and the solution diluted with 100 parts of cool water; the
solution is treated with 70 parts of 3 per cent ammonia
bichromate solution, and finally dilluted with 100 parts of
industrial alcohol. Before using, the solution is carefully
filtered. A metal plate, usually zinc, is coated with a thin
film of this cold top enamel solution by flowing, following by
spinning. After exposure in a manner similar to printing from
a photographic film, the image of developed by soaking in
alcohol, when the unaffected portions of the film are removed
and a positive print of the film is formed on the plate surface.
Since shellac is resistant to acid, it is possible to etch the photograph into relief with acid. In order to ensure that this etching is sufficiently well defined, it is usual to use a negative which has been photographed through a half-tone screen although this is not necessary if a line drawing or line diagram is being reproduced.
Start the coating pan at 38-48 r.p.m. and add just enough
shellac glaze to cover the tablets. Mix the tablets by hand to
ensure even distribution of the glaze.
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Allow
the tablets to roll until they begin sticking to the pan
and to each other. At this point, add a small amount of
dusting agent, such as talc, and again mix by hand.
When the tablets are rolling freely, blow in air at room
temperature for 5 to 10 until most of the remaining
alcohol is driven off and then dry with warm air at
29-39ºC for 25 to 40 minutes. The tablets are then
cooled and the procedure repeated as many times as
needed. Allow 48 hours of air drying before starting to
re-coat. |
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This first etch cannot be carried to sufficient depth to give
a good printing plate and a further step is necessary. This
step utilizes a specail etching powder in which the resin,
dragon's blood, is the most important ingrediant. The
plate is dipped edge first into the powder which is then
allowed to side across the plate. The powder collects against
the relief edge of shellac film, and is then firmly brushed
in, taking care that in the brush travels only in one
direction. The plate is then heated to, burn in' i.e. tifyse
abd anchor the powder. Powdering and burning in are then
carried out in a similar way but starting with another edge.
The process is repeated untill all four edges have been so
treated. The plate is then ready for its second etch in the
acid bath, after which it is carefully cleaned and washed and
is ready to go to the printer as a printer's block.
The shellac bichromate films should be conditioned in an atmosphere of constant humidity, since the amount of water present in the film affects the exposure time required. It is essential that the shellac used for this purpose is perfectly clean.
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