In the forward handlers, dustings of ground bark or other tanning material are very frequently given, and the layers only differ from these in having much heavier dusting, stronger liquors, and being allowed to remain undisturbed for greater lengths of time, ranging from a week up to a month or even six weeks, as the tannage progresses. The handler-liquors are principally from the old layers, though they are frequently made up with weak liquors from the leaches, and strengthened with extracts or gambier. Very varied materials are used in the manufacture of sole-leather. Oak-bark is one of the oldest, and as regards quality one of the most satisfactory, but it is costly, not only on account of its weakness in tannin but from the light weight of leather which it gives. Valonia is one of the favourite materials, giving heavy weight and a solid leather, in which it deposits a great deal of bloom. Oakwood, chestnut-wood, and hemlock-bark extracts are now very largely consumed, principally in strengthening the layer-liquors; the great object being not only to lessen the cost in material, but to save time, and produce greater weight and firmness. The layer-liquors in some yards where extract is used, reach strengths of even 120° to 150° Bkr. (sp. gr. 1·12 to 1·15), while in pure oak-bark yards it is difficult to get above 30° or 35° Bkr.; and even these figures are only reached by repeatedly strengthening the same liquor, in which large quantities of non-tanning substances accumulate. The opinion of the most intelligent tanners is, however, that better results are attained by a regular change of liquor, even if the apparent strength is less. When the leather has remained a sufficient time in the layers to have attained all the weight and solidity of which it is capable, it is washed up in a clear and somewhat weaker liquor or even in warm water, and taken into the shed to be dried and finished. As this finishing is almost purely mechanical, and scarcely comes within the scope of the present volume, a very brief sketch must suffice. [Illustration: FIG. 39.--Wilson’s Striking Machine.]