ISDUSTRIAL A S D ESGISEERISG CHE.TfISTRY
576
Obvioudy. by using standardized solutions of molasses and standardized apparatus to determine the amount of a reagent needed to effect a given precipitation, the above relation niay be used to estimate the "area per unit weight" factor with sufficient accuracy for the control of commercial operations. The factors entering into the problem of producing a lime powder with suitable area remain untouched in a research way, although an investigation is planned for the near future. That a problem exists is evidenced by corn-
Vol. 19, s o . 5
niercial experience wherein considerable variatioris in the "grindabilit'y" of the lime are encountered. The Raymond mill, with the usual closed-circuit air-clashification arrangement, is employed for grinding in all case5 and the maximum output per unit of equipment a t t'he different factories varies as much as *50 per cent when producing powders of equal specific surface. It mould appear at this time that grindability niay be a function of either the bource of raw material of kiln manipulation or possibly of hoth, with the raw inaterial exert'ing the greatest influence.
The Role of Lime in Tanning' By George D. McLaughlin YKIVERSITY OF CINCINNATI, CINCINNATI, OHIO
T
HE hegiiiniiig of the use of lime in connection with the
tanning of animal skin dates back t,o very ancient times, so ancient, indeed, that we have no reliable knowledge of even its approximate date. The iiiiproveinents which science has brought about in many manufacturing processes have caused the partial or complete replacement of inany time-honored materials. This is not true of the use of lime in tanning, however, which is greater today than ten years ago. Structure and Composition of Animal Skins
If we examine a stained section of skin under the niicroscope we readily see that' it' is made up of three general layers -the epidermis, corium, and flesh or adipose tissue. (The histologist splits t.hese again int,o subdivisions, but they are not important for this discussion.) The epidermis represent? probably 2 per cent of the skin thickness. The corium, which comprises the bulk of tlie skin, is the leather-forming substance. The epidermis dips down into the corium, forming a pocket or follicle in which the hair is embedded and held. Strands of elastic fibers run through the upper and lower parts of the corium, and the upper portion contains fat axid sweat glands as well as the muscles which motivate t,he erection of hairs. The whole skin is composed of mater, salts. proteins, fats, and carbohydrate. Fortunat'ely for the tanner, however, the three structural divisions are quite different in their compoeition. The epidermis is composed largely of t,he protein keratin, while t,he corium is mainly the protein collagen. Keratin is rapidly dissolved or decomposed by alkalies, whereas collagen is digested only slowly. Unhairing Process
The object of the tanner is to remove coiiipletely both hair and epidermis, as well as the flesh or adipose tissue, and at the same time to dissolve or destroy the least' amount of the collagen which composes the corium or leather-making substance. This is termed the "unhairiiig" process. The flesh or adipose tissue may be remol-ed inechanically even in t,he absence of any chemical treatment, but the epidermis must be deconiposed before it and the hair can be removed. Bringing about the proper deconiposition is one of the inost important steps in tanning. This decomposition ensues when the skin is soaked in solutions of alkalies or acids or sulfides, or when prot,eolytic bacteria are allon-ed t o act on its surface. The use of acids has never proved satisfactory, for sereral reasons, one of which is that part' of the skin fat must be saponified. Bacterial digestion is very difficult to control. There are now being marketed 1
R e c e i v e d March 21, 1927.
enzyme preparat'ions which unhair, but their use is still limited. Most unhairiiig is accomplished with an alkali, either alone or mixed with sodium or arsenic sulfide. This alkali is a saturated solution of calcium hydroxide although any soluble alkali will bring about unhairing. Advantages of Lime over Other Alkalies
Lime is favored not merely because of its cheapnein. It has a limited solubility and a saturated lime solution ha-; a p H value of about' 12.5. Therefore, the tanner may iiiake up a vat full of liiiie solut'ion and add an excess of undissolved lime, knowing that the solution's alkalinity cannot exceed pH 12.5 (at constant temperature) and that as linie i-: renioved by the skins from the solution tlie excess lime will dissolve and maintain saturation. In other words, he has a practically automatic, foolproof process. Lime does iiot digest the collagenous inat'erial of the skin so greatly as do sodiuiii and potassium hydroxides. Lime saponifies a pmtion of the fat in the skin. A lime soap is of a curdy nature, holding little n-ater. It is important that the fat in the hsir follicle be part,ially saponified so that the alkaline solution iiiay freely penet'rate the follicle and digest the keratinous niaterial holding the hair. The curdy nature of the lime soap formed permits this penetration better than the jelly-like s o d i ~ nor i potassium soaps that result' when these alkalies are used. Lime Standards
In view of our lack of fundamental knowledge of skin proteins and t,heir behavior towards electrolytes. it' is difficult to lay down strict 5tandards or specifications for liriie quality for unliairing. It cannot be doubted, however, that a high CaO content is desirable, that even a small amount of iroii is objectionable, that high inagnesium content i:, either harmful or wasteful (since it seems practically inert in re5pect to uiihairing)! and that the lime should have a high " i u s pelision factor"-that is, the excess undissolved linie added to a lime vat should settle downwards slowly. Chemical Action of Lime o n Skin Probably even more important in the unhairing process is the effect of the lime upon t,he molecular character of the skin proteins, but t,liis phase of the subject has been purposely avoided in this discussion. I t is known that the cheniical action of the lime upon the skin greatly affect. the behavior of the skin in its combination with tanning niateriala, both organic and inorganic. What chemical and phyhical changes are involved and how they may best be brought about and c,ontrolled are still unsolved problems. These highly important questions are now being investigated both in this count'ry and abroad.
