• Materials for Ceramics

CMC Gum

Alternate Names: C.M.C.

Description: GUM, Aqualon, C.M.C., C. M. C.

Notes

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.

CMC gum trade name examples are Aqualon from Hercules, Gabrosa from Alzo Nobel.

Related Information

Here is my setup to make brushing glazes and underglazes by-the-jar

To make a low SG version of G2934BL I have already weighed out a 340g batch (it contains 5g each of Veegum and CMC gum to gel the slurry and slow the drying). I use 440g of water initially (adjusting that according to experience in brushing behaviour). After shake-mixing all the powder in the plastic bag I pour it into the water on low speed and finish with 20 seconds on high speed. This produces a low specific gravity brushing glaze, it just fills this 500ml jar. In subsequent batches, I adjust the Veegum for more or less gel and the CMC for slower or faster drying. Later I also assess whether the CMC gum is being degraded by microbial attack - often evident if the slurry thins and loses its gel. Since each glaze recipe responds differently and changes differently over time, good notes are essential. We are working on dozens of these at any given time, each is code-numbered in our group account at Insight-live.com. This is so worthwhile doing that I make quality custom labels for each jar!

Common dipping glazes converted to jars of high SG brushing

These are cone 6 Alberta Slip recipes that have been brushed onto the outsides of these mugs (three coats gave very thick coverage). Recipes are GA6-C Rutile Blue on the outside of the left mug, GA6-F Alberta Slip Oatmeal on the outside of the center mug and GA6-F Oatmeal over G2926B black on the outside of the right mug). These are examples of high specific gravity brushing glazes. One-pint jars are made using 500g of glaze powder, 280g of water and 75g of Laguna CMC gum solution (equivalent to 1%). Because no Veegum is being used this blender mixes to a slurry of super high 1.6 specific gravity (SG). Commercial glazes have a much lower specific gravity (thus much more water), giving better paintability and gelling but requiring more coats. Still, this approach is good for Alberta Slip because it is highly plastic and comprises the bulk of the recipe. The gum removes the need to roast 50% of it and the plasticity of the Alberta Slip helps suspend the slurry.

Convert a pint of dipping glaze for brushing using CMC gum

We normally make a low specific gravity pint of slow-drying brushing glaze using 340:440:5:5 powder:water:Veegum:CMC proportions (this is ~1.5% of each of the gums). But where thick coverage of large areas is needed a potter can quickly convert a dipping glaze for painting. How? By blender mixing some CMC gum into it (with no Veegum the specific gravity is high enabling thicker layers). Since we mix most of our dipping glazes to about 50:50 water:powder then 340g powder is yielded by ~680g slurry. I can thus blender mix 5g CMC gum into 680g of slurry to quickly make a pint. Why no Veegum? Faster drying, and as noted, thicker coverage (still much slower drying than dipping consistency). This piece is difficult to cover using dipping or pouring (without having a large bucket of slurry). In addition, I need a thin glaze layer inside the flange and quite thick elsewhere. These issues were easy to accommodate by brushing on three coats. In fact, I can quickly glaze large 15 and 35 lb bowls, while rotating on the wheel, inside and out, using this same technique.

Gum does not work in a glaze if an important ingredient is missing

These brush-strokes of gummed glaze are painted onto an already-fired glaze (the gum enables adherence and drying without cracking). Notice the brush strokes hold their character. The brown glaze on the left has 1.6 specific gravity (SG) and about 1.5% CMC gum. The white one has the same gum content but an SG of 1.5. Its brush stroke has flattened and it is running downward. Is this because of the lower SG? No - commercial glazes with an SG down to 1.3 still hold in place well. How? Because they also have Veegum to thicken and gel them. That reveals a secret: Gum needs particle surface area to work its magic. A bentonite addition supplies that. The dried strokes on the right demonstrate that - 2% bentonite has been added to that one.

This serious glaze crawling problem was solved with a simple addition

This is G2934Y white (with 10% Zircopax). I initially blamed the zircon for the crawling. But, since the slurry had settled somewhat I was able to remove about 15% of the water and replace it with CMC gum solution. The gum addition was not enough to slow down the drying much (one reason to avoid gum if possible). That fixed it! Meaning that adherence of the dried layer to the smooth bisque was the issue. This being said, there were still a couple of small spots where it crawled. Replacing another 5% of the water should fix that. If you need to fix the problem with a gloss white it will likely require less gum, start with replacing 10% of the water.

Fixing a crawling problem with a measured CMC addition

The problem: This dipping glaze is crawling (as shown on the glazed tile). Let's assume I have already checked to make sure the specific gravity is right and the slurry is thixotropic. Because it has settled a little there is an opportunity for plan B: Remove some of the water and replace it with gum solution. Shown here is a precise calculation of the exact water content of the slurry to replace 10% of the water with gum solution. But it is better to take a more conservative and easier approach: Replace one-twentieth of the water with gum solution (too much gum and the glaze will drip excessively and dry too slowly). Rather than be overly precise let's just guess: I have 5000g of slurry that is about 50% water, so that is 2500g of powder, so I need to remove 125g of water and replace it with 125g of Laguna gum solution. A good way is to use a sponge: Wet and wring it out first and then repeat touching it to the water surface and wringing it out into a container to get 125g. A propeller mixer is needed to mix in the added gum solution (it won't just stir in).

What should the consistency of CMC gum solution be?

This is CMC 35g/liter gum solution after it has been hot-mixed (using a mixer powerful enough to put plenty of energy into the solution without sucking air bubbles) and cooled to about 30C. As it cools further and sits it will thin and can be poured. However many gum solutions have a higher CMC content, up to double this.

CMC gum solutions can go bad

That is why glazes containing CMC often need a biocide if they are going to be stored for extended periods. We made this one. The gallon jar of Laguna gum solution sitting next to this did not go bad, that means they have added some sort of anti-microbial agent.

How much CMC gum powder is in the gum solution you buy?

As it turns out, Laguna gum solution has 6.7%. We dried out 100 ml of the solution and were left with this residue on the bottom of the container.

Underglaze decoration difficult to cover with clear overglaze

The underglaze was painted on to bisque ware (has not be fired on). This is a problem. It has a high gum content and has sealed the surface so the porous body underneath is unable to pull water out to dry it quickly. During the slow dry the little absorption that is taking place is generating air bubbles from below and these are producing bare spots. The solution is to either make your own underglaze having a lower gum content or decorate ware in the dry or leather hard stage so the bisque fire will neutralize the gum.

CMC Gum is magic for multi-layering, even for raw Alberta Slip

The glaze on the left is 85% of a calcine:raw Alberta Slip mix (40:60). It was on too thick so it cracked on drying (even if not too thick, if others are layered over it everything will flake off). The center piece has the same recipe but uses 85% pure raw Alberta Slip, yet it sports no cracks. It should be cracked much worse than #1. How is this possible? 1% added CMC Gum (via a gum solution) was added! This is magic, but there is more. It is double-layered! Plus very thick strokes of a commercial brushing glaze have been applied over that. Yet no cracks. CMC is the secret of dipping-glazes for multi-layering. The downside: More patience during dipping, they drip a lot and take much longer to dry.

Six layers, 85% Alberta Slip in the glaze, yet no cracking. How? CMC gum.

Six layers of any normal dipping glaze would be impossible, flaking usually starts with the second layer. Yet this slurry is 85% plastic clay, it shrinks so much that it would be like a "dried-up lake bed" on the first layer. By the second layer it would all just fall off! How was it possible to dip six layers here? A 1% CMC gum addition (via a gum solution). Gums are often added to low-clay-content glazes to dry-harden them. But with all the clay in this one, no help is needed for hardening. This is an incredible demonstration of the power of a gum as an adhesive and hardener: It has sufficient power to actually counteract drying shrinkage! Of course, there is a downside: A drying period is needed between each layer, the length depends on the porosity and wall thickness of the ware and the amount of gum. This also demonstrates the difference between the function of Veegum (and similar materials) with CMC. The former, if added to this recipe, would gel the slurry, require more water and drastically increase the shrinkage, making the cracking even worse. Of course, one could simply use a mix of calcine:raw Alberta Slip to control drying shrinkage and gum would not be needed.

CMC gum in the water is preventing this powder from slaking

Normally the powder would slake and settle to the bottom immediately. Mixing this requires a powerful mixer and plenty of time to remove all the lumps. The proportion of gum in the water amounts to 1.5% the total weight of the powder in the glaze.

CMC Gum ruined a batch of this glaze

I used a mix 95:5 water:gum solution when making this glaze. But even that small amount of CMC gum slowed down absorption into the bisque (and drying speed) enough that it was impossible to get it on thick enough. The version of the glaze used for the mug on the left has no gum, it is a thixotropic slurry of 1.45 specific gravity. A 3-4 second dip of a bisque piece is enough to build up a good thickness, which is important for this glaze. It dries for handling in less than 10 seconds. The one on the right was dipped twice for about 8 seconds each time and it took a minute or so before it could be handled. Yet it is still much too thinly applied. This glaze needs at least some gum because it is crawling slightly, it is an example of where precision is needed to determine the maximum that can be tolerated without impacting application thickness.

Gum content in glazes can have a dramatic effect on drying time

This is a low fire brushing glaze. It has been sitting on this plaster bat for two hours and shows little sign of dewatering. A typical pottery dipping glaze, by contrast, would dewater in seconds! Clearly, such glazes are only good for brushing.

Veegum CER Saturated suspension

This is the most viscous suspension we can make, 300ml water with 13.25g Veegum CER (50:50 mix of Veegum T and CMC Gum). It required the use of hot water and our two-gallon-mixer, on its highest speed, mixing in this cup. Add this gel to a raw glaze to improve its brushing properties and slow down drying. Often this works better than CMC gum alone because it gels the slurry enabling increasing the water content and improving suspension properties. Or, substitute this for part of the water in low-clay-content recipes (if the glaze is already viscous, just add this). In slurries having sufficient clay use pure CMC gum instead (unless a high-water-content slurry is needed).

Crawling in G2934Y Zircopax white glaze: Here are some fixes.

G2934Y, a variation of the G2934 base, is a good stain matte base glaze but it is not without issues. It has significant clay content in the recipe and high levels of Al₂O₃ in the chemistry, these make it susceptible to crawling. This base is normally fine as is but when opacified or certain stains are added (especially at significant percentages) it can crawl. This has 10% Zircopax. Even though the glaze layer thickens at the recess of the handle join it is still crawling. We also get this on the insides of mugs where wall and foot meet at a sharp angle. This was initiated because the glaze cracked here during drying. Normally it would heal but the zircon stiffens the melt, making it less mobile. The easiest solution is to adjust the specific gravity of the glaze to 1.44 and flocculate it to thixotropic, this assures application is not too thick. Another measure is to add a little CMC gum (by replacing some of the water with gum solution). Lastly, use a blend of tin oxide and Zircopax, as in the G3926C version of the recipe, to opacify it.

Control gel using Veegum, brushing properties with CMC gum

This is G1214Z1 brushing glaze (with 5% titanium added). For a 340g powder batch (to get a pint) my initial target is 5g CMC gum and 5g Veegum. CMC controls drying speed and Veegum the amount of gelling. I first mix the CMC with the powder and shake the whole batch in a plastic bag. Then I add it all to 440g of water in the blender jar and mix it well (making sure no agglomerates remain (stage 1). Stage 2 is adding the VeeGum slowly, while high-speed blender-mixing, this is important because it enables tuning the degree of gel (which cannot be predicted). Because this recipe has little clay, it took all 5g of Veegum without overgelling (the entire mass moved freely in the mixer jar). But it did gel overnight (so 4g would be better next time). By contrast, it is the brushing behavior that demonstrates whether the amount of CMC is right. Not enough and coats dry too fast and go on too thick. Too much and it dries too slowly and too many coats are needed.

What to do when glazes drain and drip like this

These are the same dipping glaze, it is variegated and drips look ugly on fired ware. The slurry of the one on the left had a specific gravity of 1.45, it was creamy and appeared to be good. However, when this bisque porcelain mug was pulled out after the dip and rolled it did this (this is ugly in the firing). Why fix the slurry instead of just rubbing this out? Keep reading. To fix this I added water to decrease the specific gravity to 1.43. Then I added Epsom salts to induce thixotropy and gel it back to a creamy consistency. This time it went on evenly. Although the piece dries enough to handle in less than 30 seconds, it does take longer to dry completely because there is more water in the slurry. An addition of CMC gum would also slow down drying but it would be very tricky to get just the right amount (otherwise dripping could be worse than this). Consider a few of the advantages of thixotropic dipping glazes: They don't settle like a rock in the bucket, they don't go on too thick or in multiple thicknesses, they don't drip and they don't dry too fast.

Paint another layer onto a fired glaze? Yes. With CMC gum.

The cone 6 mug on the left has the G3933A glaze, applied as a dipping glaze. It turned out poorly - crawling from corners and looking thin and washed out. I made a brushing glaze version of this (which adds 1.5% CMC gum), I keep it around for this very purpose. It has a high specific gravity (unlike commercial ones that have high water contents - they will run and go on too thin if you try this). Because of the gum it dries hard, there is no shrinkage or cracking. On a second firing, using the C6DHSC schedule again, (mug on the right) the surface is transformed - thicker, more vibrant color (being picked up from the underlying body).

Make frit melt fluidity test balls using CMC gum as a binder

Traditionally we have made GBMF test balls of non-plastic materials (e.g. frit) using Veegum as the binder. However, Veegum interacts with materials enough to affect melting. CMC gum is better. I use 10g frit (the right amount for one ball), 0.1g CMC (1%) and ~3g of water. Of course, you need a scale accurate to 0.01g to be able to weigh out only 0.1g (otherwise consider doing 50g). The procedure that seems to work best is to shake the frit/CMC mix in a small bag, pour it into a bowl, pour in a tiny amount of water and stir and smear with a spoon. With the right amount of work and water the thickened material will become cohesive and can be formed by hand. Does cornstarch work? No, it does not gel like this and the mix is not plastic either. Psyllium? Yes, but it has a flakey texture and demands more water. For better formability use 1.5% CMC gum - however, the ball will dry slower and much harder. Other powders will behave differently (needing more or less water, forming better or poorer, dewatering slower or faster). You will need a plaster surface to absorb the excess water (which invariably happens).

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