Four additives that can transform the way you glaze
Glaze formulations differ depending on how they are intended to be applied. Dipping and brushing have different requirements and single coating glazes differ from those intended to be multi-layered. Much of the difference is down to additives. In this article I introduce four such that can be used to give excellent results with the most common application techniques.
CMC Gum
Sodium carboxymethylcellulose, known as CMC is an organic gum used as a glaze hardener and viscosity modifier. It is usually supplied in granular form which can be used directly in the glaze but it is better to mix it with water first to create the additive that I call ‘CMCSOL40’. This is prepared by mixing 40g of CMC powder into 1 litre of water giving a liquid that has a similar consistency to wall paper paste. Mixing by hand is possible with patience but much better results are achieved using a power mixer or spinning the mixture for several hours on a ball mill rack. Higher concentrations of CMC are possible but they require specialist mixing equipment. CMC is subject to attack by micro-organisms. This is only a problem if it is stored for several months and can be remedied by adding a very small amount of a copper salt, for example 0.1g of copper sulphate per litre of water.
CMCSOL40 has the mass fractions:
F(water) 0.962
F(CMC) 0.0385
Vee Gum T
Vee gum T and Bentone are extremely fine grained refined clay minerals from the bentonite/hectorite family. ‘VGTSOL80’ is prepared by powermixing 80g of Vee gum T with 1L of water. It forms a semi-solid paste with the consistency of soft margarine. Thus prepared, it is easy to disperse into a glaze. In contrast, mixing powdered Vee gum T into a glaze is very difficult. Veegum T is inorganic and therefore not subject to attack by microorganisms.
VGTSOL80 has the mass fractions
F(water) 0.926
F(Veegum) 0.0741
Deflocculant
My deflocculant of choice is the sodium polyacrylate sold in Europe under the trade name ‘Dispex’ and in North America as ‘Darvan’. In glazes it is superior to more traditional deflocculants such as sodium silicate as it is very tolerant of over use and is able to deflocculate a mixture lacking any clay, at least in part due to the steric effects of the polyanion.
Flocculant
My preferred flocculant is a saturated solution of calcium chloride prepared by adding approximately 70g of the anhydrous salt to 100ml of water at room temperature. The exact concentration is not critical. Many potters use magnesium sulphate (Epsom salts) as a flocculant, this also works well.
Sources
CMC, Displex and calcium chloride are widely available form pottery supply companies in the UK. Vee gum T is more difficult to source but is currently (2024) available from Scarva Pottery Supplies in Northern Ireland. Bentone is currently (2024) offered by CTM Potters Supplies.