CMC Powder (Sold in .25 Units)

Safety Data Sheet

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The term CMC is generic and refers to organic sodium carboxymethylcellulose. In ceramics, this material is added to glazes, colors and enamels. As percentages are increased CMC gum makes slurries stickier and slower drying (these properties increase exponentially with percentage added). In industry, where ware exiting driers can be quite hot, slowing down drying is important to maintain at least some moisture to improve adhesion of the next layer (of engobe, glaze, grit, etc).

CMC gum is organic, so microorganism attack can affect slurry rheology (so an anti-microbial might be needed). Also, note that organics generate gases on firing, these can produce glaze defects.

CMC gum is commonly used in glaze slurries to improve dry hardness and adherence to the bisque. It is especially important in brushing glazes for another reason: Slower drying. Gum is an important addition to over-glaze colors applied by stamping or painting, it gives them a paint-like consistency. Gum is important in transparent glazes that need to adhere and spread evenly over underglaze decoration. Highly fritted glazes benefit most from gum additions because they lack clay content to harden and suspend them. Conversely, with a gum addition, high-clay glazes that would otherwise crack on drying (and thus crawl during firing) can dry without problems and adhere to bisque. Single-layer dipping glazes that have natural hardening properties, because they contain 15% or more clay, normally do not need gum additions (unless there is a need to slow down the drying speed). Base coat glazes (applied by dipping and covered with subsequent layers) need gum to firmly adhere them to the bisque (to prevent being lifted because of the shrinking of subsequent layers).

When enough CMC gum is present it becomes the dominant factor determining slurry rheology, even negating the effects of other rheology modifiers. A glaze that formerly relied on water content to control viscosity will likely require a specific gravity adjustment when CMC gum is added (often less water).

Powdered gum can be very difficult to disperse in water (addition-as-a-powder to an existing slurry is almost impossible). While it is possible to mix gum powder with other dry ingredients before adding them to the water (often 0.5-1.5%) a much more effective method is to make a gum solution (gumsol) and replace part of the water with it when making a batch. Make the solution by boiling water and mixing in the powder vigorously with a mechanical mixer (it should thin out over time). What percentage of gum should be in a gum solution? We use Laguna Gum Solution, it works well and contains 67 grams of powdered gum per litre (as well as an antimicrobial preservative).

The amount of solution needed in a glaze must then be calculated to source the amount of gum powder desired. We find that about a 5:95 gumsol:water mix is good for our G2926B base coat dipping glaze, but this amount does not necessarily apply to others (testing is needed to be sure the gum amount is not excessive). Brushing glazes are at the opposite extreme, requiring as high as 20:80 gumsol:water. It is normal to allow a glaze slurry to settle and remove an amount of water equal to the amount of gumsol being added. That being said, gumsol may thin the slurry, so it might be better, for example, to remove 200g of water and replace with 100g of gumsol.

Do not substitute something else just because it has the word "gum" in its name. VeeGum T, for example, is a refined plastic clay, a completely different type of material (CMC gum is a glue, Veegum T is a high surface area clay that encourages gelling in glaze slurries lacking fine-particled clay in their recipes).

There are reasons to use a mix of VeeGum T and CMC Gum. For example, it is often advantageous for brushing glazes to have a low specific gravity (e.g. 1.3), however such high-water-content slurries are like water. Veegum T acts as a thickener and CMC gum a hardener. There is a material named "Veegum CER", it is a mix of CMC and Veegum T, it is used for just such a purpose. Since VeeGum CER thickens or gels the slurry, it is practical to add it, in solution, to an existing dipping glaze (without having to remove the same amount of water). A VeeGum Cer addition can mean that more water is also needed to get the degree of gelling needed.