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Re: [BKARTS] alum-tawed skin on oak boards: chemistry



Hello, Nancy,

The black spots are ink; specifically, iron gall ink.  The combination
of iron, tannic acid, and moisture makes the ink.

Just guessing here.  Alum-tawed skins are harder on skiving knives
than vegetable tanned leather, so it is not uncommon to touch up the
cutting edge of the knife during the paring operation.

If the knife is not cleaned very throughly after sharpening, and if
you don't wash your hands with soap and water after sharpening, and
if you sharpen on the same bench where you are paring, iron particles
are going to be involved.

All it takes, then, is moisture.  Moisture can come from the paste and
also from sweat on your hands, and in a few minutes there are black
measles on your oak boards.

Wearing latex gloves only removes one source of moisture and will make
it more difficult to handle the leather, and will do nothing about the
iron particles.

Hope this helps.

Jack



>Hello,
>
>I am a new member of the list, and stumbled here because of a problem I've
>been having with alum-tawed goatskin on oak boards. After poking around the
>web and writing to some other binders, I found Jack Thompson's post from
>1998 (
><http://palimpsest.stanford.edu/byform/mailing-lists/bookarts/1998/09/msg000
>7>
>http://palimpsest.stanford.edu/byform/mailing-lists/bookarts/1998/09/msg0007
>7.html) about the reaction of tannins in oak boards and moisture working to
>create ink spots. This is *exactly* what has been happened to 3 of my books,
>and it happens most noticeably on the corner turn-ins.
>
>My question is about the exact chemistry involved. Is something from the
>alum-tawed skin permeating the wheat paste, and the paste acts as a vehicle
>and interacts with the tannins from the oak to create the spotting effect?
>I've never had this problem with other leathers on oak boards, so I must
>assume that the alum-tawed skin contributes something to the chemical
>reaction. Jack's post refers to sweat from hands as well--is this a factor?
>If so, will wearing latex gloves help impede this reaction? Will keeping the
>amount of paste to an absolute minimum help? I'd really like to know the
>precise chemical reaction, if possible.
>
>Many thanks in advance for your help!
>
>Best wishes,
>Nancy Hulan

Thompson Conservation Lab.
7549 N. Fenwick
Portland, Oregon 97217
USA

503/735-3942

http://www.teleport.com/~tcl

"The lyfe so short; the craft so long to lerne."
Chaucer _Parlement of Foules_ 1386

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