The processes employed in the production of leather with the vegetable tanning materials vary extremely according to the class of leather which is being produced, both in the materials selected and in the time required. In sole-leather tanning, where thick hides are used, and where diffusion is the only force acting to carry the tannin into the hide, many months are frequently needed, while with thin skins, and with the aid of mechanical motion, which circulates the tanning liquid between the fibres, the process is often complete within a few hours. Differences in the strength of the liquors according to whether hard or soft leathers are to be produced, and the mutual action of the acids naturally present in the liquors, and of the tan, have also a determining effect upon the quality of the product. The simplest form of tanning in principle is probably the old-fashioned method of sole-leather manufacture. For this purpose, the hides are usually “rounded” or trimmed after liming, unhairing and fleshing, so that the most valuable part, the “butt,” can be tanned separately from the “offal.” The butts are usually washed in water to remove a portion of the lime, considerable care being required at this stage to avoid carbonation and fixation of chalk by means of free carbonic acid, or hydric calcium carbonate (temporary hardness) in the water employed, or by the free carbonic acid of the air. This somewhat primitive process can at best only remove a small portion of the lime, since so long as the lime remains in the caustic condition, it is very obstinately held by the hide-fibre. Advanced tanners now frequently employ weakly acid baths, in addition to washing, in order to produce more complete deliming, and this effects a very considerable improvement of colour in the early liquors. The use of lactic acid (free from iron) or boric (boracic) acid in solutions of about 4 lb. per 100 gallons, in which the butts are kept in motion, are among the safest and most satisfactory ways of removing surface-lime and improving the colour, but even the stronger mineral acids may be used successfully with caution (see Chap. XIII.).