The paper pulp obtained from straw consists of a mixture of short fibres together with a large proportion of oval-shaped cells. The fibres are short and somewhat resemble esparto, but the presence of the smaller cells is a sure indication of the straw pulp. The fibres themselves closely resemble the fibres of esparto, but as a rule the latter are long slender fibres, while the straw fibre is very often bent and twisted or slightly kinked.
Fig. 27.—Straw.
The only method of distinguishing between straw and esparto is by examination with the microscope. There is no chemical reagent known which will produce a colour reaction on a paper containing straw that will serve to distinguish it from a paper containing esparto. If such papers are gently heated in a weak solution of aniline sulphate a pink colour is slowly developed, the intensity of which is to some extent a measure of the amount of straw or esparto present.
Straw and esparto are usually described in text-books under one heading, partly because the fibres possess strong resemblances in physical and chemical constitution, and partly because the methods of manufacture are identical. At the same time the qualities of the two pulps are so different that they cannot be used indiscriminately, the one for the other. Straw cellulose cannot be utilised in the place of esparto, particularly for light bulky papers. Hence in magazine and book papers containing a fibre which gives a pink coloration with aniline sulphate it is fairly safe to assume that esparto pulp is present.