Re: Tannic Acid

Thanks Ryuji

I presume that I have condensed tannins as it wasn't very soluble. Is there
anything I can try to make it more soluble, such as heat for example?

John

www.johnbrewerphotography.com

----- Original Message -----
From: "Ryuji Suzuki" <rs@silvergrain.org>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Saturday, November 12, 2005 8:40 AM
Subject: Re: Tannic Acid

> From: John Brewer <john@glossyormatt.com>
> Subject: Tannic Acid
> Date: Fri, 11 Nov 2005 21:06:37 +0000
>
>> Cheaply and easily available is wine tannin. Is there a way this can
>> be used to tone cyanotypes as tannic acid? My first foray today was
>> hopeless. The method I used was to acidify a dry print in acetic
>> acid, wash, bleach in ammonia, wash, soak in water with some tannin
>> in. Googling doesn't seem to be much help. Is tannin also tannic
>> acid? If not can I make tannic acid with tannin? The tannin I have
>> is a fine brown powder.
>
> Tannin is a very broad class of polyphenolic compounds, and tannic
> acid does not mean a specific acid or any specific compound but it
> also referrs to a broad class of polyphenolic compounds. (To make it a
> bit more confusing, "tannic acid" is often used regardless of a
> particular compound being proper acid or not.)
>
> There are two major subclasses of tannin. One is condensed, and the
> other is hydrolyzable.
>
> Condensed tannins are the ones naturally precipitated as insoluble
> polymer in fermentation and aging of wine. This class of compound is
> characterized by 2 to 8 flavan-3-ol or flavan-3,4-diol groups in each
> molecule. There, smaller polyphenolic groups are covalently bonded and
> they are not readily hydrolizable by merely dropping the pH. This type
> of tannin is not very reactive in aquaous media because they are not
> very soluble.
>
> Hydrolyzable tannin are gallotannins or ellagitannins. One common
> example is pentagalloylglucose, where 5 gallic acids are bound to a
> glucose. This is an ester and the binds can be readily broken down by
> lowering the solution pH. This form of tannin does not naturally
> precipitate in wine making so it is often removed by adding fining
> agents (gelatin, albumen, and other forms of edible proteins, or
> polyvinylpyrrolidone, particularly in the case of white wines. Yes,
> the PVP is the same stuff as the one used in Kodak HC-110 developer as
> the antistaining agent.) and filtering out the precipitated solids. But
> those tannins are bound to the fining agents and they are not usable.
>
> If you got cheap "wine tannin" mostly consisting of condensed tannins,
> it makes sense to me that it is not very effective. If you want to
> test if your tannin solution is active, you might want to do a quick
> test by adding a few drops of weak gelatin solution in a beaker of
> tannin solution (no warmer than room temp). The gelatin should
> precipitate out.
>
>
> Incidentally, if you make a cup of green/black tea and leave it out,
> you'll probably see haziness in several hours. This is caused by
> oxidation of tannins. Bottled tea products often contain ascorbic acid
> to prevent this haziness. (Black tea is probably easier to cause this.)
>
> Some of the very fine wine drinkers may find some wine also develop
> haziness, called casse. The same oxidation of wine tannins can be
> involved but presence of iron and cupper (from the grapes) can
> complicate the thing. Adjusting tannin content in wine strictly by the
> "art" is tricky as the chemical composition of wine varies. What's
> worse, grapes that are infected by fungus cause more trouble due to
> enzymes brought in with the infected grapes.
>
> Vintners treat grapes with sulfur dioxide before crushing, use fining
> agent to reduce the tannin content, and add metabisulfite, citric
> acid, EDTA, etc. to inhibit the transition metals and enzymes, all to
> prevent casse (and also improve other sensory qualities of the wine).
Received on Sat Nov 12 08:23:13 2005