wallace myers on wed 19 nov 97
Would some kind soul please tell me;
1) what is cmc gum used for,
2) exactly how do you use it,
3) does it spoil if mixed too long?
Thanks, Wallace
Martin Schiller on thu 20 nov 97
In a message dated 11/19/97 2:39 PM, wallace myers said....
>1) what is cmc gum used for,
It's a white powder that is soluble in alcohol or water, and when
disolved, can be mixed into a water based glaze mixture. It thickens the
glaze and keeps the particles in suspension. It also causes the glaze,
after it is applied, to be harder and to have a semi permeable quality.
This makes handling and applying overglaze decoration to the glazed ware
easier.
>2) exactly how do you use it,
>3) does it spoil if mixed too long?
Exactly how is a matter of personal preference. Apparently, dissolving
the stuff is a process that causes folks to want to mix up a large batch
at a time. I found that CMC would dissolve in alcohol rapidly and could
then be added to glazes on an as needed basis (didn't spoil.) Doesn't
seem that I added more than a couple of tablespoons to a 10 lb glaze
batch. But that was a long time ago, and others have lots more experience
with measurement.
regards, Martin
Cupertino, Calif
Tony Hansen on fri 21 nov 97
An organic sodium carboxymethylcellulose that acts to harden unfired
ceramic glazes (cement the particles together) for safer handling of the
ware. Highly fritted glazes (lacking clay content) used in factory
settings benefit greatly from the addition of gum. Gum additions are
often unnecessary if the glaze has natural hardening properties (i.e.
from 20% or more clay). However gum is an important addition to stain
mixes that are applied over-glaze by stamping or painting.
Gum can help suspend and stabilize glaze slurries somewhat, however
other additives (an adequate amount of the right clay in the batch) are
more effective.
An important side effect of gum additions is that they cause slower
drying. While this is advantageous for brushing glazes, it can make it
very difficult to achieve an adequate glaze thickness and prevent drip
marks. Dipping glazes work best if they are both naturally thixotropic
and quick drying. Both of these properties can be detrimentally affected
by gum additions.
Veegum CER is a 50:50 mix of CMC and Veegum T.
Powdered gum can be very difficult to disperse in water thus it is
difficult to add it to an existing liquid batch. However if gum powder
is mixed with other dry ingredient before adding them to the water it
can be done (often 0.5-1.5%). A much more effective method is to boil
water, add about 25-30 grams of powdered gum per litre and mix
vigorously with a mechanical mixer. This mixture must be added during
mixing to replace part of the water. It is difficult to set a standard
proportion because the amount of gum needed is totally dependent on the
glaze's ability to harden. A starting point for glazes that powder or
smudge excessively might be 1 part gum solution to 3 parts water.
Performance of the mix compared with the side effects can then be
evaluated and the proportion adjusted.
--
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T o n y H a n s e n thansen@digitalfire.com
Get INSIGHT, Magic of Fire at http://digitalfire.com
Fay & Ralph Loewenthal on sat 22 nov 97
I mix 35 grams of CMC with 1 litre of water. If you find this
too thick then reduce the grams to 30. I use 500 ml CMC
liquid / jelly mixed with 1 litre distilled water and 1 litre
propylene glycol. I mix in this mixture into our underglazes
and stains. My wife loves it and suggests we start
marketing it. BTW my stepson got the recipe from
somebody on Clayart. They said this liquid together with
some other feldspars etc made up the medium for Mason
Stains. How correct this is I do not know. We only use the
liquid mixture after testing the complete formula. We find
the colours come out brighter with just the liquid. I usually
use 15 grams of underglaze powder with 20 ml of liquid. It
depends on the strength of the colour. Some blues I only
use 10 grams. Hope this helps Ralph in PE SA
Tadeusz Westawic on sat 22 nov 97
wallace myers wrote:
>
> ----------------------------Original message----------------------------
> Would some kind soul please tell me;
> 1) what is cmc gum used for,
> 2) exactly how do you use it,
> 3) does it spoil if mixed too long?
>
> Thanks, Wallace
Canvas artists use gums as a pigment-carrier. I forget what it is;
cellulose-something.
1) mix it with water or as per instructions in a painter's reference.
2) let it sit for a day or so until lumps are out.
3) add it to a glaze that has dry peeling, cracking or flake-off
problems.
There was recently on this list a good post about mixing CMC. I was
surprised that I didn't see CMC mentioned in the peeling-flaking dry
glaze thread in the past week. Maybe I missed it.
I have heard people complain of a decomposing stink. I get that in
glazes in which I use it, but not in the pre-mixed bucket of CMC itself.
On the safe side, don't mix more than you can use in a week or two. Two
to five pounds of the stuff should be enough for you to experiment to
your heart's content for several months.
Other gums (tagaranth, e.g.) are used in similar fashion. I assume that
acrylic gloss medium is similar.
CMC can be used as a transfer medium. I have been able to lift iron
oxide patterns off steel with it. I have also been able to lift oxide
patterns off rocks with it. Applying the resulting "decal" has been a
little "testy", but the technique holds promise. There is a problem of
scale, lifting large areas with the gum. I'm pretty sure one can
silkscreen with the stuff.
In the kiln, CMC burns-off at about cooking oven temperature, before red
heat. You might see a scorching brown or black color on your ware as
this happens. There are fumes in early firing stages, I don't know what
they are but I avoid them to be safe.
CMC can attack a soft bisque, causing some spalding type damage. This is
especially so if the lumps are not out of the gum before you use it.
Best to fire the ware within a few days after application.
Fresh mixed CMC looks like a bucket of snot, you can chase the kids with
it.
Tadzu
Israel Amirav on mon 24 nov 97
>----------------------------Original message----------------------------
>I mix 35 grams of CMC with 1 litre of water. If you find this
>too thick then reduce the grams to 30. I use 500 ml CMC
>liquid / jelly mixed with 1 litre distilled water and 1 litre
>propylene glycol. I mix in this mixture into our underglazes
>and stains. My wife loves it and suggests we start
>marketing it. BTW my stepson got the recipe from
>somebody on Clayart. They said this liquid together with
>some other feldspars etc made up the medium for Mason
>Stains. How correct this is I do not know. We only use the
>liquid mixture after testing the complete formula. We find
>the colours come out brighter with just the liquid. I usually
>use 15 grams of underglaze powder with 20 ml of liquid. It
>depends on the strength of the colour. Some blues I only
>use 10 grams. Hope this helps Ralph in PE SA
************************************************************
Hello Ralph,
I did not follow the discussion on this issue but from what I understand
,you are talking in your post about some kind of media that you mix under
glazes with.
I have been looking for a long time for a media that will work nicely with
stains to formulate u.glazes,but had no success up to now.
Can you give more details about this issue.Is there a recipe available ?
Thanks,
Naama in Israel
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