The principal function of fatliquoring is to influence the degree of fiber cohesion which takes place before drying. If there were no cohesion whatsoever, the skin would separate into its constituent fibrils, leaving no leather structure. On the other hand, if all the fibrils and fibers cohered, the skin would then take the form of a hard and horny material having no value as leather. Somewhere between these two extremes there is an ideal degree of cohesion for any given purpose for which the leather is to be used.
The fatliquoring process probably acts to control the differential shrinkage of grain versus corium (dermis) of the leather during drying, thus playing an important role in controlling the degree of tightness ofBREAK of the leather. In addition, it also influences the handle, drape, flexibility, durability, stretch, and water resistance of leather, and also adds greatly to its strength. These are factors of primary importance in leather used for covering books.
Fatliquoring is usually carried out in a drum at the highest temperature practical for the type of leather, or about 45° C. for vegetable tanned leather and 60 to 65° C. for full chrome tanned leather. The skins are run in a drum for 30 to 40 minutes. After drumming, the leathers are usually struck out on the flesh side, carefully set out to smooth the grain, nailed or toggled out flat to dry, or paste dried.
To allow a small amount of oil to be spread uniformly over the very large surface area of the leather fibers, it is necessary first to dilute the oil. Although this could be done by means of a solvent, e.g., benzine, it is more economical, safer, and more convenient to emulsify the oil. In an EMULSION with water, the oil is dispersed in microscopically small droplets. It is important that the drops of oil in the water remain as an emulsion until they penetrate the leather, and not separate out as large drops or as a layer of oil, which could not penetrate the leather fiber and result in merely a greasy surface layer.
The properties of the finished leather can be varied by controlling the degree to which the emulsion penetrates before it "breaks," depositing the oil on the fibers. Relatively shallow penetration, which leaves the inner layers of the leather with comparatively little oil, gives a leather that has a tight break and is soft and resilient. On the other hand, if the oil is allowed to penetrate uniformly through the leather, the result will be a leather that is soft and stretchy, with any natural grain looseness accentuated.
The liquors incorporated into the leather are called fatliquors, of which there are several types. One of the earliest was the soap fatliquor, consisting of neatsfoot oil and common soap. When a vegetable-tanned leather is drummed in such a fatliquor, the slight acidity of the tannage neutralized the soap, causing fine drops of oil to be deposited on the fibers. The greater the acidity of the leather, the more rapidly this occurs, so that the oil may be deposited before it has penetrated sufficiently. Consequently, soap fatliquors are described as having poor emulsion stability, and are generally used where a fairly heavy surface fatliquoring is required. It is possible, however, to modify many types of oils chemically so that they become miscible with water without the use of emulsifying agents.
Sulfated oils are used frequently because they give good, fine-oil dispersions and are less sensitive to acid than soap fatliquors. This results in deeper penetration of the oils into the leather before they are deposited. Sulfated oils are prepared by treating fish, animal, or vegetable oils with sulfuric acid at a temperature of 10 to 20° C. The resultant product is washed with a strong brine solution to remove excess acid. The salt is necessary to prevent the sulfated oil from emulsifying with the water. Soda ash is then added to form the sodium salt of the sulfated oil and to neutralize the last traces of the acid. The more the oil is sulfated, which is to say, the more sulfuric acid that has been fixed, the greater will be its stability to acid and the more thorough its penetration into the leather. Conversely, the more acid the leather, the less the penetration. However, increasing the amount of sulfation or water miscibility, decreases the "oiliness" of the oil and therefore its lubricating powers.
Sulfonated oils are prepared by a similar process, usually at a higher temperature; the fatliquor contains the sulfonic group which gives greater stability and emulsions which penetrate deeper into the leather under acid conditions.
Still another method of obtaining emulsifying properties is to sulfite the oil. In this process, the oil is mixed well with a strong solution of sodium bisulfite (NaHSO 3 ), while the mixture is thoroughly aerated by means of compressed air. Sulfited oils behave in a manner similar to sulfated oils, but are usually said to be more acid stable and to afford deeper lubrication.
In addition to the emulsifying element of the fatliquor, a raw oil, such as mineral, castor, neatsfoot, cod or coconut oil is frequently included in the formulation. See also: CURRYING ; STUFFING (2) . (248 , 275 , 291 , 306 , 361 , 363 )