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Re: Tanned bronze



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          CENTRAL NEW YORK BOOK ARTS: TRADITIONAL TO INNOVATIVE
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I am new to the bookbinding world but use stains for other things. In "MY"
experience bronzing is an oxidative processes. The metal components in the
dye come out to create the bronzing. I imagine that some leathers absorb the
dye more effectively and these probably are not oxidized as easily because
the dye is distributed throughout. While those leathers that only stain the
surface create a "layer" of dye that is more easily oxidized. Again this is
extrapolations from my experience NON BOOK/LEATHER related.

Alex

-----Original Message-----
From: Book_Arts-L: READ THE FAQ at http://www.philobiblon.com
[mailto:BOOK_ARTS-L@LISTSERV.SYR.EDU]On Behalf Of David Lanning
Sent: Thursday, January 03, 2002 7:47 AM
To: BOOK_ARTS-L@LISTSERV.SYR.EDU
Subject: Re: Tanned bronze


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          CENTRAL NEW YORK BOOK ARTS: TRADITIONAL TO INNOVATIVE
           See The Exhibition Online, And Order Your Catalog.
                      <http://www.philobiblon.com>
             ***********************************************

> Hello All,
>    Does anyone know what causes the "bronzing"
> effect when some leathers are redyed? I have read that
> high concentrations of aniline dye powder will cause this,
> and that certainly seems to be true, but I have noticed
> bronzing with very weak concentrations in water,
> in ethanol, and in methylated ethanol. The effect seems most
> pronounced on Oasis goat with water-soluble dyes, and least
> noticeable on fair (undyed) calf using Fiebing leather stains;
> apart from that, the bronzing seems most unpredictable.
> I wonder if it might have to do with the tanners' finishes on the
> skins...
> Any thoughts would be most welcome; any cold hard facts
> would be great too.
> Thankya!
> Bill Cotter


Dear All,

Bill is of course quite correct in stating that a high concentration of dye
in water will cause 'bronzing'.  However, there may be other factors that
should be considered.

'Oasis' and similar leathers such as our Clansman Nigerian Goat, are
drum-dyed.  They may then be surface dyed by the manufacturer to obtain a
better match to the desired colour 'master'. The surface dyeing in our case,
is done by spraying dye on to the grain-side of the skin.  The amount of dye
applied in this spraying process depends on the base colour and the amount
of adjustment needed.  i.e. some of the leathers in the range will have more
dye sprayed onto the surface than others.  There is a limit to the amount of
dye that may be applied before the leather is saturated.  Beyond this point,
further dyeing by the manufacturer or the bookbinder, no matter how weak the
concentration would possibly cause 'bronzing'

So the possible scenario here is that manufacturer has applied a substantial
amount (maybe to the leather's saturation limit) of surface dye, and then
the bookbinder applies further dye.  This additional dye is above and beyond
the limit that the leather can handle and the dye just 'pools' (stays) on
the surface.

An answer to this problem would be to add a small amount of milk to the dye
solution.  This would help prevent the 'bronzing' effect.  However, one of
the side effects of this solution is that the adding of the milk (protein)
may prevent the dye from correctly bonding with the leather.  The result is
leather where the dye is not 'fast'.  A suitable fixative would need to be
applied to prevent colour coming loose while handling.

It should be noted, that water based dyes which have had no spirit/alcohol
added, will only work on water permeable leathers, i.e. those which do not
have a water-resistant finish. Most of the top-end bookbinding leathers
produced by ourselves and our competitors do have water-permeable finishes.

kind regards,

David


David Lanning
J. Hewit & Sons Ltd.
sales@hewit.com
http://www.hewit.com
Tel: +44 (0) 20 8965 5377
Fax: +44 (0) 20 8453 0414

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