The linen fibre has an average length of 27 mm. with a diameter of ·02 mm. The raw flax is very different from raw cotton and is easily distinguished. The fibre is slender in shape, having thickened knots at regular intervals throughout its length, the general appearance of which may be compared to a stick of bamboo. The central canal of the fibre is extremely narrow, running like a small thread through the length of the fibre. The cell walls are further marked by numerous pores, which appear as small dark lines running from side to side, but not meeting in the centre.
Fig. 19.—Linen.
In the treatment necessary for making paper these characteristics are largely destroyed, and while it is quite easy to ascertain that a paper is of linen, or of cotton, or that a paper is mainly cotton with a small percentage of linen, yet there are conditions under which it is difficult to determine the exact percentage of cotton or linen in a rag paper. If, for example, a paper contains nearly equal quantities of cotton and linen, the exact proportions cannot be determined closer than 10 per cent., especially in well-beaten papers.
Reagent for Staining Fibres.
Preparation.—Dissolve 2·1 grams potassium iodide and 0·1 grams iodine in 5 c.c. of water. Mix this solution with a solution containing 20 grams of dry zinc chloride in 10 c.c. of water. Allow the mixture to stand; pour off the clear liquid into suitable bottles.
Coloration Produced.
Cotton, linen, hemp.—Wine red.