[alt-photo] Re: evaporation of gum mixture

[Paul Viapiano]

I think what I posted about is relatively simple and if I had thought about 
it enough I wouldn't need to have posted.

Let's say we leave a pigment/gum mixture out overnight in a dish. It 
inevitably hardens. What gets evaporated to cause this?

It must be the water in the gum arabic mixture, regardless of whose gum it 
is or where the "tears" came from.

The same thing is happening at a much slower rate in my plastic container 
which is not airtight. That's all.




----- Original Message ----- 
From: "Tomas Sobota" <tom at sobota.net>
To: "The alternative photographic processes mailing list" 
<alt-photo-process-list at lists.altphotolist.org>
Sent: Sunday, July 04, 2010 9:07 AM
Subject: [alt-photo] Re: evaporation of gum mixture


> Some months ago I posted here about an experiment I made with gum talha
> (Acacia seyal). I received a small quantity from friends in the Sahrawi
> camps in southern Algeria, where the tree also grows.
>
> I prepared and used this gum just like senegal, with totally disappointing
> results: all the gum coat seemed to harden and hence, no image.
>
> This test was just a fast experiment, so I cannot swear that some other
> variable didn't get in the way. Now that you remind me, I will try again 
> in
> more controlled circumstances.
>
> Gum senegal in stones (you call them "tears" but what is sold here is so
> large that it could only be called crocodile tears) is very easy to get
> here, and reasonably inexpensive, that's one of the reasons that I buy it
> instead of prepared gum. Also, the work is not that much: the gum 
> dissolves
> easily during a night or less.
>
> Tom Sobota
> Madrid, Spain
>
> On Sun, Jul 4, 2010 at 5:25 PM, Katharine Thayer 
> <kthayer at pacifier.com>wrote:
>
>>
>> On Jul 3, 2010, at 1:22 AM, Tomas Sobota wrote:
>>
>>  I have never used prepared gum solution. I prepare gum in the original
>>> artisanal way, dissolving gum arabic stones in water. Then, I keep the
>>> solution in dark glass bottles, well stopped and preserved with a few
>>> thymol
>>> crystals. Kept in this way, I have never observed any evaporation, not
>>> even
>>> in a time span of years. So, what is being described is strange, and
>>> probably related to the way that the commercial gum is prepared or
>>> preserved.
>>>
>>
>> Well, it may be more than that.  I've expressed my suspicion before that
>> the Daniel Smith premium gum may be partly or wholly a different acacia
>> variety (acacia seyal)) since it behaves differently from other gums I've
>> used, not only in this tendency to dry out  but in other ways that
>> correspond to the description of acacia seyal.  I tried to verify or
>> disconfirm this suspicion with Daniel Smith but couldn't get an answer 
>> from
>> them.   But Kremer, for example, does (or did, last time I looked) 
>> specify
>> that some of their gum arabics are  a mixture of acacia senegal and 
>> acacia
>> seyal, or even acacia senegal OR acacia seyal.  Like Daniel Smith,  other
>> suppliers (Bostick & Sullivan, Photographer's Formulary) don't specify 
>> the
>> acacia variety used in their "gum arabic" (apparently there's no 
>> requirement
>> that "gum arabic" be acacia senegal.
>>
>> I agree, mixing your own from tears is probably the best way to get the
>> right kind of gum; the reason I don't  has been laziness I suppose, not
>> wanting to bother with the effort of searching out a supply of good 
>> quality
>> tears of the right variety in the right grade (maybe this is more easily
>> done on your side of the ocean?) combined with the ease of use of
>> commercially prepared gum.  I did buy some powder from Daniel Smith and 
>> mix
>> it, but if their prepared gum isn't entirely acacia senegal, the powder
>> probably isn't either.  I haven't had an opportunity to use that gum yet,
>> since I've been in the hospital three times in the last year for
>> heart-related surgeries and explorations and have been deeply involved in
>> another project in the meantime.
>>
>> Curious, I started searching online for gum acacia senegal in tear or 
>> lump
>> form, just to see how difficult it would be to take this approach, and 
>> found
>> that it's not that simple. I went down three pages of google hits and 
>> found
>> basically wholesale distributors with minimum orders of from 100 kg to 
>> 6000
>> kg, and in places like China.  In this group I found several distributors
>> who list their A. senegal as grade 1 and A. seyal (aka "gum talha") as 
>> grade
>> 2 or 3, FWIW.    I also  learned that the US imports largely processed 
>> gum
>> and European countries impor largely lump gum, processing the gum and
>> reselling it to the US in processed form.   Which might explain why it's
>> easier to find lumps of gum in Europe than it is in the US.
>> Katharine
>>
>>
>>
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