The washing of preserved rawhides and fur skins before tanning has the purpose of removing dirt, blood, dung, preservatives, fat and water-soluble proteinacous compounds. This is done mostly in a paddle tub or in the tanning tumbler. Frequently service water is used which has a hardness of about 15.degree. dH (degrees german hardness). The washing and cleaning solutions employ the following aids:
(a) Surface-active substances of an anionic or nonionic nature, which not only clean the skins, but also improve at the same time the feel of the fur.
(b) Fat solvents in emulsified form, such as hydroaromates (hydrogenated aromatic hydrocarbons) or petroleum-hydrocarbons.
(c) Inorganic salts, like common salt, which help to improve the washing effect of the tensides by an electrolytic effect. A high pH of 8.5 or over is undesired and can lead to damage of the skins and rawhides. The use of the inorganic salts leads to a high salt content of the waste waters, which is subject to increasing criticism.
A further problem in washing rawhides and skins, particularly when treating raw materials with a high fat content, is that the cleaned product can become fatted again (refatted) and the apparatus can be contaminated in the dilution stage on rinsing. In other words the suspended fat tends to deemulsify and deposit on dilution during rinsing. An improved stabilization of the wash liquors is thus desirable. Furthermore questions of sewage treatment and the resulting costs are becoming important and will become more and more important in the future.