When mixed with water it makes a gum. In ceramics it is used to make brush-on glazes. It can be used as general-purpose binder with underglazes and stains.
CMC is an organic compound. When mixed with water it makes a thick liquid (less viscous), the less water you use the thicker the gum, the more water the thinner.
We use it in our Vitraglaze range of brush-on glazes, with a water mixing ratio of 20g of CMC powder to 1 Litre of water.
If you put your 20g of powder into a pan, add the water (1 Litre). Then bring up to a high heat on a hob, you do not want to boil it but almost get to that point, then turn off. If you boil the water you will lose water content and this will affect the calculation. Allow to cool and break up the clumps with a fork. Put a lid on the pan and let it absorb overnight.
Place in a sealed container, if possible one which is right for the size of the amount and do not keep for longer than a few days, so only make as much as you will need in the short term.
When you have mixed up the CMC with the water as instructed above, use the newly created mixture as your water ingredient in the glaze. A typical range for a powdered glaze is 700ml/800ml to a 1kg of powder. Each powdered glaze should be adjusted to suit your own personal application requirement and for the nature of the powder in the glaze. It is advisable to start at the lower end of the mixing ratio then increase as necessary. Don’t forget to sieve when thoroughly mixed.
CMC is hydroscopic so it will absorb water from the air, both in powder form and liquid. Keep it well sealed from the air.
It is a perfect medium for growing life, put it into clean jars and keep it out of sunlight.
But if it does, it will not harm a glaze although the smell would be unpleasant. You can retard this by adding a biocide, such a tea tree oil or anti-microbial agent.
CMC’s are different, there are many brands of CMC. Some have different strengths, particle/flake sizes and grades. If you have bought CMC from another supplier please seek their advice for their best mixing ratio.