Stephen Lathrop on wed 25 aug 99
The question was posted what is the difference between CMC and Veegum. CMC and
Veegum are rather different in their chemical structures and moderately
different in their functions. There most common applications are in glazes, but
may be used in clays. In water both thicken and become jelly like (colloidal or
gelatin) and appear to offer a similar result in a glaze slurry. A discussion of
the mechanisms at the molecular level may not have as much use. But, in
understanding the application and performance differences might be more helpful.
I should note that there are a number of CMC like materials. And there are lots
of different Veegums, but they are as groups very different. I do not have my
supplier catalog for Veegum so I can't look up the specific one you mentioned.
As binders and dispersants they are used throughout cosmetics, art products,
ceramics, and consumer products. Veegums have been around a long time while CMC
has only seen wide spread use since the 60-70's when its manufacture was
perfected.
CMC is an organic and Veegum is an inorganic. CMC is like things made of plant
and animal tissues, tissue materials generally burn out during firing because it
is carbon based. Veegums are inorganic and less effected by low temperature
changes. Veegums are like our inorganic glaze materials in that they do not
normally burn off and can and do become part of the melt.
To go a little further CMC (Carboxy Methyl Cellulose) is more similar to the
gelatin, it sets up a very firm gel in water. Then when that gelatin/water
material is dispersed in a glaze mix it disperses as a lot of individual jelly
particles or non scientific term globs. The globs are can be seen if the glaze
is pressed between two plates of glass. So the gelatin particles are relatively
large by microscopic standards. CMC both reduces settling physically in the
bucket and hold glaze ingredients together when drying. In the glaze on the pot
CMC acts as a kind of binder with glaze particles and binds to the pot as well.
This binding or sticking mostly takes place below the surface of the glaze
coating sticking glaze ingredients together more in a macro microscopic fashion.
CMC will break down from bacteria action over time. If you use it could use an
inhibitor, use it up in a relatively short time, or replenish it if its effect
diminishes.
Veegum (a colloidal Magnesium Aluminum Silicate) is all mineral based and acts
more like a clay. In water Veegum is colloidal so it thickens the water just
like CMC does but, in a somewhat looser, weaker, smaller fashion. Its binding
power is at the molecular level or very microscopic and not as easily observed
in glaze. Veegum colloids make water appear as if it were thickened like CMC,
but still more flowable or movable. So it will take much more Veegum than CMC
if it is being used for dispersing in the glaze bucket and more if it is being
used as a binder to hold the glaze on the pot. Veegums have a tendency to form
films and in glazes form the film on the surface of the glaze when it dries, not
within like CMC. Veegum is generally much weaker so if it is doing the same job
as CMC it requires much more to obtain a similar dispersing effect or binding
effect.
Helpful hints for preparing a CMC concentrate to add to glaze. Powder CMC
absorbs water so strongly that it has a nasty habit of sticking to itself in
large clumps during preparation. To minimize this, use cold water only for
mixing. Finely sprinkle in the CMC powder into cold water while mixing
vigorously. If in a hurry, heat the mixture to near boiling to help devolve the
reaming clumps. Do not boil as it has a tendency to foam. Recommend and prefer
to let the mixture stand over night to completing the desolving of any remaining
clumps.
Any day with clay is a good day.
Steve
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