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Re: Bookbinding Leather Suppliers

Lester writes:

>Peter brings up vegetable v. chrome tanning and Rick mentions acidity
>as possible problems with cheap leather.

It's not quite as simple as that (unfortunately!).  Even in the conservation
literature, I get the impression that what makes some leathers more durable
than others in not well understood.

Whether a leather is veg-tanned, chrome-tanned, or alum-tawed, it is going
to be acidic.  That's just the nature of the tanning process.  However, alum
tawed skins seem to have lasted better over the centuries than veg-tanned

>   The Tandy "senior calf" I bought to learn on is vegetable tanned and
>no thicker than the Harmitan Nigerian goat I got from a book supplier.
>As to what Tandy calls "book leather," it looks just like what Peter
>described as spray painting a split to disguise the defects.
>   I pH tested both Tandy's calf and Harmitan's goat by mixing scrapings
>from each skin into pH neutral water. After some time I tested the water
>with ColorpHast pH indicator strips. Both came up very acid. The tests
>were neither controlled nor identical so I cannot say one is more acid
>than the other -- only that Tandy's "senior calf" and Harmitan's
>Nigerian goat >>appear<< to be in the same acidity league.

I'm not sure that this is a valid way to test for the true acidity of the
skin since it assumes that the acids are not bound to the skin but are free
to go into solution.  How acid is 'very' acid?

With veg-tanned skins, an important factor in how long the leather lasts is
the TYPE of tannin used.  There are two broad classes, the catechol (sp?)tannins
and the pyrogallol tannins.  The former class seem to be more prone to degrade
leathers over time than the latter.  Oak and sumac contain pyrogallol tannins.

Then there may be questions about what chemicals were used at all steps of
the process (unhairing, pickling, bating,etc.), what fats/oils/waxes may have
been used in finishing, dyes, etc.

cheers, Rick

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