104 Chapter 5
With the replacement of formaldehyde with such polymerising agents as
urea, dimethyl methylene and ether and the introduction of a more complex
variety of aromatic base groups, a very wide range of synthetic tanning mater-
ials (syntans) have become available commercially.
The tanning reaction of syntans is partially due to the presence of hydroxyl
groups forming hydrogen bonds in a similar manner to vegetable tannage, but is
mainly a result of the sulfonate groups forming electrovalent bonds with the
free amino groups on the collagen.
3 The Manufacture of Leather
The first leathers were probably made by our hominid ancestors using oils
and fats, possibly in conjunction with a smoking stage. It has been speculated
that scraping the skin impregnated the structure with subcutaneous adipose
material giving a softer, more long-lasting product. This led to other fatty
products from the animal, such as brains and marrows, being used. These con-
tainboth unsaturated fatty acids and natural emulsifying agents resulting in
the production of a true leather. It has also been speculated that it may have
been observed thatskins used to cover tents in which smoking fires were lit
gradually developed further desirable properties and that this resulted in the
introduction of a separatesmoking process.
Common salt (sodium chloride) has been used to preserve food since earliest
times. It is, therefore, quite possible that naturally-occurring alum was first
employed to treat skins when it was mistaken for common salt. The special
properties of these particular crystals were noted and exploited for making
CH 2 CH 2OH OH OH OHnnOH OH OHCH 2 CH 2SO 3 HH 2 SO 4HCHOSO 3 HFigure 9Outline of the Novolac method for syntan production