When sulphite or soda pulps are used, either separately or conjointly, in the manufacture of printing papers, the quantity of neither material shall separately exceed 50 per cent.
The most complete specification as to the requirements for standard papers is that published by the Paper Testing Institute in Germany, and used as the basis of most contracts, at least for public and official documents.
Standards of Quality in Germany.—The classification of papers according to the raw materials used and the nature of the finished paper is very complete. The classification is made under three headings: (A) Raw Material; (B) Strength; (C) Uses.
(A) Classification according to Material.
(1) Paper made from rags only (linen, hemp, and cotton).
(2) Paper made from rags with a maximum of 25 per cent. of cellulose from wood, straw, esparto, manila, etc., but free from mechanical wood pulp.
(3) Paper made from any fibrous material, but free from mechanical wood pulp.
(4) Paper of any fibrous material.
(B) Classification according to Strength.