PESCULIN (Fr., iEsculine ; Ger., ZEskulin, Schillerstog) Synonyms, esculine, esculin, esculinic acid, polychrome, bicolorin, enallachrom. An ex tract obtained from the bark of the horse chestnut (iEsculus Hippocastanum). C15H1609. Molecular weight, 340. A white powder, a solution of which, of a strength of about i part in 50o parts of water, is used as a filter to absorb ultra violet rays.
In process work, where the white (particularly Chinese white) reproduces as if it were yellow, an sculin filter should be used. This may be a solution contained in a glass cell having parallel sides, or it may be in the form of a dry filter.
OR (Malayan) (Fr. and Ger., Agar-agar) A gelatinous vegetable material obtained from certain white seaweeds (Gracilaria lichenoides and Eucheuma spinosum), found principally on the shores of the Red Sea. It is used to a small extent as a substitute for gelatine in plate and paper making, but it is more difficult to melt than gelatine, and is not generally so satis factory for emulsion work. It has also been recommended as a substitute for arrowroot in the preparation of silver paper, the latter, if very porous, being first sized with a ri per cent.
solution of gelatine. Five parts of agar-agar are allowed to soak for an hour or two in 50o parts of water, heated till dissolved, boiled for five minutes, and then mixed with twenty parts of common salt and strained through a cloth. It is carefully brushed on the paper, and this, when dry, is floated in the dark upon a 14 per cent. solution of silver nitrate containing io per cent. of citric acid. In the case of thick and coarse paper, the silver solution is first brushed on, and then, when dry, the paper is floated on the agar-agar solution and again dried in the dark. After being sensitised the paper keeps well. It is printed and toned as other plain silver papers, but if platinum is used as a toner the picture must be deeply printed.
In process work, agar-agar has been suggested as a substitute for fish glue in the enamel pro cess of preparing the resist for etching, but it has not come into commercial use.