Retanning

1.0 Stiasny

1.1 Stiasny started the research
1.2 He produced Neradol D which was later found to be a non-swelling acid which would wash out of leather. It is an auxiliary syntan.
1.3 Syntans are now a complex and expanding subject. In the 1960's we looked at three types:

a. Auxiliary
b. Replacement
c. Sulph

2.0 Auxiliary

2.2 These cannot tan themselves
2.3 Are strong acids, (pH 1.7-2.0)
2.4 Hasten veg tan, dissolving quebracho
2.5 Increase the tensile strength of leather
2.6 Increase the ability of the leather to take up grease
2.7 Some have pH up to 3.6, which usually means they have been partly neutralised.
2.8 Tan: non tan ratio varies considerably
2.9 Ash contents very variable. Important to check as only a certain amount of ash is allowed in some leathers.
2.10 7-10% solution can be used for bleaching, dipping hides in momentarily.
2.11 2-4% on pelt weight normally used in tanning.
2.12 With veg tans they

a. restrict mould growth
b. prevent sludge
c. speed up tan
d. give more economical use of tan

2.13 Used early in the process they increase the penetration of the tan.
This gives less fixation in the grain and reduces the likelihood of cracking leather.
2.14 Will remove iron stains
2.15 Aid to rapid drum tannage of sole leather
2.16 Do not affect the weight giving properties of veg tan (important as sole leather sold by weight)
2.17 In light leather used with kips and sheep etc.

a. clear the colour
b. disperse veg tan
c. to get improved tightness of the belly (sometimes)

2.18 Essentially additives

3.0 Replacement Syntans

3.1 can tan on their own
3.2 Give veg tan type
3.3 More expensive than veg
3.4 can replace veg with a change in process
3.5 Only used when props are superior to veg
3.6 pH 3-3.5
3.7 tan: non tans higher than auxiliaries
3.8 Very variable colour. White to pure yellow to brown.
3.9 Unless specified the white is unstable to light
3.10 Some have mild bleaching properties
3.11 Some have mild dispersing properties
3.12 Tannage varies from mellow to firm
3.13 Gives lighter colour than veg
3.14 Gives tighter grain than veg
3.15 Gives better dyeing properties than veg
3.16 20-25% (on veg tan) in sole leather and 25-30% (on veg tan) in harness leathers.

4.0 Sulph Tan

4.1 Really replacement Syntans
4.2 developed before replacement syntans.
4.3 Particularly important sulphur, and others which give white tannage which is fast to light.
4.4 generally expensive
4.5 Used on ewes, bucks, reptiles
4.6 Used largely along with chrome
4.7 pH 2.2-2.4 at 10% dilution
4.8 Props vary widely. Look for light fastness, staining with iron, salt sensitivity, bleaching of iron stains, softness/hardness.

5.0 Drawn Grain

5.1 Frequently syntans are used to draw the grain (e.g. for shrunken shoulders)
5.2 Buffalo is given characteristic grain by drawing
5.3 For drawing/shrinking grain

a. water content kept down
b. astringency can be increased by adding formic acid
c. normally a pretan before chrome
d. if to be dyed do not use expensive white syntan)