[alt-photo] Re: evaporation of gum mixture

[Katharine Thayer]

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