1902 Encyclopedia > Parchment

Parchment




PARCHMENT consists of skins of various animals, unhaired, cleaned, and dried so as to form sheets of uniform thickness suitable for writing upon and for the numerous other purposes to which such preparations are devoted (see PALAEOGRAPHY, p. 144). The skins employed for parchment are principally those of sheep, lambs, and calves; but goat and ass skins are similarly dressed for special purposes. The preliminary unhairing and cleaning of the skins are effected as in the leather manufacture (see LEATHER, vol. xiv. p. 380). In their moist flexible condi-tion the unhaired skins are tightly and uniformly stretched over a wooden frame termed a herse, and on the flesh side they are carefully gone over with a semicircular fleshing knife which removes all adherent flesh. The grain side is also gone over to clean the surface and squeeze out a proportion of the absorbed moisture. Ordinary binder's parchment and drum-head parchment need no further pre-paration, but are simply allowed to dry gradually on the frames on which the skins are stretched. But fine parchment for writing and vellum are powdered with chalk on the flesh side and carefully rubbed with fine pumice stone till a delicate uniform velvety surface is raised. All inequalities on the grain side are also re-moved by paring and rubbing with fine pumice. Stout vellum is made from calf skins, and ordinary qualities from split sheep skins; for drum heads, tambourines, and like applications goat and calf skins are used, and it is said that wolf skins yield the best drum heads.

Vegetable Parchment, or parchment paper, is a modified form of paper produced by chemical treatment, having considerable similarity to ordinary animal parchment. It is prepared by acting on ordinary unsized paper with dilute sulphuric acid, and immediately washing away all trace of acid. Paper so treated will be found to have undergone a remarkable change : the porous intertexture of cellulose composing unsized paper will have expanded and agglutinated, forming a homogeneous surface, translucent, horny, and parchment-like ; it will have acquired about five times the strength of ordinary paper; it will become soft and flaccid when steeped in water, to which, however, it is impervious ; and it is unaffected by boiling in water. The formation of vegetable parchment is due to a molecular change in cellulose when acted on by sulphuric acid, owing to which the substance is transformed into a starch-like body—amyloid—with simultaneous swelling of the fibres, which thereby soften and agglutinate. The preparation of vegetable parchment was patented in 1857 by Mr W. E. Gaine, and machinery has been adapted for the manufacture. The paper to be acted on passes in a continuous web through a vat containing commercial sulphuric acid diluted with half its volume of water. In this it is immersed from five to twenty seconds at a tem-perature of about 60° Fahr. It then passes in succession through pure water, next an ammoniacal solution to remove all acid, and finally again through water, after which it is dried and finished by passing between felted rollers and over heated polished metal cylinders. A similar effect is produced on paper by treating it with a syrupy solution of zinc chloride at from 120° to 212° Fahr. Vegetable parch-ment has not realized all the expectations of it. It is most largely used as covers for preserve jars, bottles, &c, and to some extent for tracings of plans, charts, &c.






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