The Good, The Bad, and The Ugly
Below is an article written by Dan Freeman. President of the Custom Boot & Shoemaker's Guild. Reprinted from The Harness Shop News 3/97
Factors Affecting Leather Quality
An animal's diet controls the quality of
the skin or hide. Undernourished animals produce thin hides, but you also can get too much
of a good thing: a diet which produces fat animals, especially the artificial fattening
used for beef cattle, can result in too much fat content in the hide. This can prevent the
lime and tannins from penetrating the hide.
Seasonal influence affects hide quality so much that cattle hides bring different prices, depending on the time of year they are slaughtered. Animals killed in July, August and September, for example, have more hide and less hair than other times of year.
When all the natural characteristics affecting hide and skin quality are considered, sex is probably the least important. The skin of the female is thinner, but usually has slightly more tensile strength. Bull hides are generally somewhat heavier than steer hides.
Hide parasites can also ruin leather. Warble flies burrow through cattle hides, most densely in the prime kidney area of the hide. Hide worms (nematodes), grubs and ringworm likewise destroy hide substance and reduce leather value accordingly.
Even if the hide or skin is "perfect" from nature, it is in danger from many sources of mechanical damage. Branding produces an expensive defect, whether it is done with the traditional hot iron, freezing or chemically. Easy visual identification is very important to cattle raisers, and a large, clearly readable brand is worth more to him than the slight loss of value of his raw hides.
Damage to hides and skins can occur after slaughter but before curing. Improperly bled carcasses can retain blood in the hide, discoloring it. Flaying that produces "flesh cuts" can seriously damage a hide. The fleshing machine is precisely adjusted to remove the fatty flesh from the hide; let it get out of tune, and it can cut up the hides or leave flesh that interferes with tanning. If the hides are dragged along the floor, the grain may be abraded, leaving a dull finish. Hides cannot be allowed to freeze; the ice crystals which form break the fibers, weakening the leather; stains can also result from freezing.
Delaying the cure has probably destroyed more hides and skins than any other bad practice. Like meat, hides start to decay the moment the animal dies. The first sign of this is a "hair slip". Hides damaged by decay start to liquefy, and the hair surface slips around on the rotting hide. The fibers can also become fixed; that is, glued together by the deteriorating gelatin, collagen and keratin present. If tanned, such skins and hides will have hard, stiff areas. If no worse damage shows up, hides cured too late will at least show some dulling of the grain.
Hides and skins, promptly cured, avoid having these problems. Curing is usually done by a dry salt, wet salt, or brine soak process. This leaves the hide in a stable condition, allowing it to be stored until it can be shipped to the beam house. Curing, storage and shipping, however, provide their own dangers.
The brine cure, in which skins or hides are paddled in a vat of brine, takes 16 - 24 hours, and has generally replaced the salt pack cures which take three to four weeks. This process does not make leather or even rawhide. It merely prevents bacterial action from destroying the hide before it reaches the beam house or tanner. If the skins or hides are too crowded in the brine tank, salt cannot penetrate all parts of them and decay commences.
Brine draw, the dehydration of hide substance in the brine tank, is a problem that did not occur when hides were salt pack cured. It occurs because the epidermis is semi-permeable: water can go through it but salt cannot. Since brine has an affinity for water, water can be drawn out of the hide and no salt will penetrate. If all steps are taken correctly, brine draw will not occur, but overcrowding, too high a temperature in the brine, inadequate fleshing of the hides, and too-dry hides can result in uncured spots in the hide and weak or grainless spots in the leather.
Some cured hides and skins go directly to tanneries to be manufactured into leather. But many go first to a beam house. There the hides are limed and split before proceeding to a separate tannery. "Leather is made in the beam house" is a common sentiment among tanners, and it is true that top quality limed hides produce top quality leather if tanned well. However, even if the cured hide reaches the beam house in perfect shape, it can be damaged there.
Lime, of course, is a strong alkali. Insufficiently diluted, it can burn the hide badly, first causing case hardening of the hide, and, in extreme circumstances, removing the grain entirely. Calcium soaps are sometimes formed when free fatty acids present in the hide are neutralized by the lime. This can result in untanned areas. Under-soaking cured hides before immersing them in the lime can result in differing degrees of chemical action on different parts of the hide. This can cause uneven stresses which can seriously weaken the fiber structure of the hide.
Unhairing and fleshing are done in the beam house and, if the delicate machinery that does these jobs is not working correctly, the hide can be damaged by cut, holes, and thin spots.
Preparatory to tanning, the hides are pickled and, if needed, degreased. Pickling involves lowering the hides pH so the tanning solution can permeate it ~ carelessly done, it can leave acid burns on the hide. If the hide is poorly cured, the acid can plump up the middle while starting to seal the outside, resulting in "double hiding" or leather that splits into two pieces, both worthless.
Vegetable tanned leather can suffer from a "raw" or untanned center caused by two types of error. If the initial tanning solution is too strong, it will tan the outsides of the hide and seal off the center which remains raw. The same effect results from leaving the hides in the vat, pit, or drum for an insufficient time; the tannins never reach the center. This defect is clearly visible in the finished leather.
Excessive mechanical agitation can result in loose, flabby leather which has had too many of its fiber bundles broken down; this condition can also be caused by improper pH balance between the hide and the tanning solution.
Grain discoloration (staining) is mostly a problem with vegetable tanned leathers which are left in their light-colored, natural state. It may be caused by overcrowding in the tan vats (which results in "kiss spots" where the hides have been touching each other), grease stains (from the animal or machinery), and metallic stains that occur when wet vegetable tanned leather touches iron resulting in black stains almost immediately.
What To Look For
Considering all that can go wrong during the tanning process, it may seem a wonder that any decent leather is ever made. But there are many competent and careful farmers, slaughterhouses, beam house and tannery operators, and there is much excellent leather available. What should we look for when choosing leather? How to pick the good and avoid the bad?
If the leather feels hard and horny, it probably will not flex and work as you would like. If it is soft and "raggy", it will stretch too much and fail to hold its shape. Hard leather indicates too much fiber coherence, and loose leather indicates no cohesion at all.
Look for a drawn grain by looking obliquely at the leather surface, letting the shadows indicate the flaw.
Drawn grain is the opposite of smooth leather. While fat wrinkles on the neck are sometimes unavoidable, wrinkles elsewhere can mean trouble. Note, however, that there is a big difference between the type of raised grain that is made on purpose by boarding (box grain) or enzymatic action (shrunken grain) and the wrinkles in the grain caused by manufacturing error.
When the grain is dull or uneven, it probably will not improve with work or conditioning. Again, when this is intentional, as in Nubuck, it is uniform and planned for; isolated dull areas, however, indicate grain damage.
Cracky grain is usually a bad sign. If the grain cracks open when the leather is bent double (flesh side to flesh side), it most often indicates case hardening (tanning of the grain, leaving the middle raw) or tender grain, weakened by one or more of the bad practices previously listed.
Making The Grade
Most of the serious problems encountered in leather manufacture leave traces on the finished product. If you cannot find any of the flaws listed so far in a particular hide or skin, it is probably a good one. It will do the job and give satisfaction.
You can learn a lot from a close examination of a cleanly cut cross section of the leather. Two things to look for when evaluating leather are raw (untanned) streaks in the middle and dye penetration. Although struck through (dyed all the way through) wears the best, dyestuffs are very expensive, and sometimes you will have to settle for excellent leather with an undyed (not untanned) center section. But watch out when the color is only a thin skin on top; black can be maintained to wear well but colors cannot.
Smell the leather: it should smell good. A strong chemical smell indicates powerful solvents or chemicals in the finish, common but not so good; it usually means a solid pigment (like paint) finish. An aniline finish is similar to wood stain - it shows the leather much better. A smell of petroleum suggests mineral oils were used to adulterate (or even replace) the animal or vegetable oils with which the leather was fat liquored. Unlike vegetable and animal fats, mineral oils do not form chemical bonds with the leather fibers and will settle like water in a sponge.
Uniformity is your last criterion. Is the temper uniform throughout the skin or hide, taking into account the natural variations? Are the color and finish uniform? On pigment dyed leather, the color should be perfectly uniform; aniline leathers will show some color variation that varies with density.
Remember, if it looks good, feels good, and smells good, the leather will allow you to make products--whether it's shoes, saddles, or handbags--that you can be proud of and which will give your customers years of satisfactory service.