These products can be divided approximately into three classes:—
(1) Papers coated on one side or both sides with various substances, such as “art,” photographic papers, etc.
(2) Papers impregnated with chemicals, such as blue print, medicated, and cheque papers.
(3) Paper pulp converted into modified products by chemical treatment, such as vulcanised board, viscoid, etc.
Of the first class, the coated papers used for art and chromo illustrations are the most important.
Of the second class, the blue prints and papers impregnated with chemicals, chiefly employed for the production of engineers' drawings, may be regarded as typical.
In the third class, vegetable parchment and vulcanised board are the most familiar.
Parchment Paper.—This is produced by the action of sulphuric acid upon ordinary paper, the most suitable for this purpose being made from unsized cotton rag, free from such additions as mechanical wood pulp. The presence of the latter substance should be avoided, as it is liable to char or burn, so that in the finished product it shows itself in the form of small holes. The process depends upon the power of sulphuric acid to change the surface of the paper into a gelatinous mass, which has been shown to consist of a substance called amyloid.
The best parchment is made from pure cellulose such as rag or chemical wood pulp. The quality of the parchment depends upon attention to the strength of the acid, the temperature of the acid bath, the period of immersion, the complete removal of the acid, and the careful drying of the wet parchment.