Preparation for Tanning Leather
A majority of folks think that tanning is the way animal hides are turned into leather. Although it is a key step in the transformation, the tanning step actually gets performed fairly quickly and without much fanfare about 3/4 of the way through the process.
The first step performed after the hide is removed is called “fleshing”. It’s a fairly straightforward process involving sharp knives wielded by folks with strong constitutions.
That was the old days. Today it’s done by a big nasty machine which is totally misnamed. One would thing a fleshing machine would put flesh on…it doesn’t. Excess flesh and fat is removed and the hides are subjected to some method of curing. Layering with wet salt or soaking in brine are common ways used to preserve the hide prior to shipment to the tannery. This removes moisture and prevents the hides from rotting while being shipped to tanneries worldwide.
Who needs hair anyway?
Hides are rehydrated and washed in huge rotating drums before removal of fur or hair. Dehairing usually accomplished by a chemical digestion process using strong alkaline like solutions including lime and sodium suphide with some mechanical agitation. Several treatments with acids and enzymes follow the dehairing process to neutralize and soften the hides.
Leather happens by a couple of different methods of tanning. Socalled “vegetable tanning” is the original prehistoric method still used with a few updates involving mechanization to decrease the time involved. It still takes up to 4 days! This method gives rise to the word “Tanning” from the active ingredient in the plant material called “Tannin”. Vegetable tanning is used to produce heavier leathers for items like shoe and boot soles and saddlery.
The other common method of turning hides into leather is “chrome tanning”…still called tanning even though the process doesn’t involve any tannin whatsoever! Hides are soaked in a solution of trivalent chrome in rotating drums until the chrome completely penetrates. This may take up to 8 hours. Chrome tanning is generally used for finer leathers like shoe uppers and handbags.
Just one more thing!
OK! Three or four small things. The wringing operation lowers the moisture content of the hide by various means foul or fair to allow the next step. If you’re old enough to remember the wringer on mom’s washing machine, imagine a HUGE one with felt on the rollers. The hide is split then shaved down to the required thickness for the end product…shoes, handbags, leather jackets, gloves and chaps. What do you so with the split part that doesn’t have a smooth skin on it? That’s where suede comes from!
I guess at this point, since tanning has been accomplished, we can say we have leather even though it isn’t particularly pretty or useful. It just isn’t rawhide any more.
Huge rotating drums figure prominently in the next process sometimes called “Fatliquoring”. A mixture of hot water dyes, fats, synthetic oils and added tanning agents (the secret sauce) is used to introduce lubricants and color for the intended end use of the leather.
The “Setting” operation removes most of the water that was used in the fatliquoring step. No huge drums involved here…just a relaxing stretch on the rack with thumbscrews or the leathermaking accessories called toggles.
“Staking” is a massaging operation that uses various machinery to soften the leather by flexing it. This brings to mind a horrific adventure Mike Rowe of Dirty Jobs had with a staking machine (pardon the ad). This can also be accomplished by tumbling in yet another huge drum. And there are a lot more automated modern processes that don’t involve risk to life and limb. Just like the one on the right.
the big finish!!
Up till now, it’s all been preparation work like all the cutting, joining, gluing, smoothing and sanding you do to a piece of furniture before the stain and poly.
Various surface finishes are added to improve wear and weather resistance. Other finishes can change the texture, color and gloss depending on the demands of the fashion of the day. It seems like there’s a whole branch of science devoted to making leather look like vinyl!