The term CMC is generic and refers to organic sodium carboxymethylcellulose. Gums are used in ceramics to harden unfired ceramic glazes. This makes for safer handling of ware, better adherence to the body, slower drying (thus better brushability). CMC gum also reduces drying shrinkage and enables multi-layering without flaking or cracking. Highly fritted glazes (lacking clay content) used in factory settings benefit greatly from the addition of gum. Single layer cover glazes that have natural hardening properties (i.e. from 15% or more clay) do not need gum. Gum is an important addition to stain mixes that are applied over-glaze by stamping or painting.
In dipping glazes, gum can act as a suspending agent (but this is seen as a side effect, there are much better ways to suspend slurries). CMC gum makes slurries stickier, more difficult to clean off tools and containers. CMC-containing slurries do not respond to gelling (via epsom salts or calcium chloride) so the main way to control the thickness is by adjusting specific gravity. This makes it difficult to tune the rheology of dipping glazes that need CMC gum (to enable them to handle over layering). The slower drying makes it difficult to achieve an adequate glaze thickness and prevent drip marks during dipping.
Veegum CER is a mix of CMC and Veegum T.
Powdered gum can be very difficult to disperse in water (almost impossible to an existing liquid batch). While it is possible to mix gum powder with other dry ingredients before adding them to the water (often 0.5-1.5%) a more much effective method is to make a gum solution (replacing part of the water) when making a batch. Gum, added as dry, may simply not work. Make the solution by boiling water (up to 70 grams of powdered gum per litre) and mixing vigorously with a mechanical mixer (it should thin out over time). The amount of solution needed must be calculated to source the amount of gum powder needed. Sometimes people advise adding gum solution to an existing glaze slurry to fix some issue, however to supply enough gum to help would raise the water content far too high.
Organic binders need to burn away so of course they can cause some problems (e.g. pinholing in glazes). They should burn away in such a fashion that the particles of mineral and frit are drawn into contact with each other to encourage reaction and prevent crawling.
Depending on time, temperature, pH, gum can be attacked by microbes or molds. If this happens store in a cooler place, make smaller batches, adjust the pH to make a less friendly environment, or add a biocide (i.e. Tektamer, NaN3). The shelf-life of commercial brushing glazes can be affected for this reason.
You cannot necessarily substitute something else just because it has the word "gum" in its name. VeeGum, for example, is a refined plastic clay, a completely different type of material (CMC gum is a glue). If the CMC was being added to a non-plastic mix then VeeGum would work better, but if a glaze already has plenty of clay material then adding VeeGum will increase drying shrinkage (e.g. make a cracking-on-drying problem worse).
CMC gum trade name examples are Aqualon from Hercules, Gabrosa from Alzo Nobel.
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.
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.
Common dipping glazes converted to jars of brushing glazes
These are cone 6 Alberta Slip recipes that have been brushed onto the outsides of these mugs (three coats). Recipes are GA6C Rutile Blue on the outside of the left mug, GA6F Alberta Slip Oatmeal on the outside of the center mug and GA6F Oatmeal over G2926B black on the outside of the right mug). One-pint jars were made using 500g of glaze powder, 75g of Laguna CMC gum solution (equivalent to 1 gram gum per 100 glaze powder) and 280g of water. Using a good mixer you can produce a silky smooth slurry of 1.6 specific gravity, it works just like the commercial bottled glazes. Amazingly, the presence of the gum also makes it unnecessary to calcine the Alberta Slip.
How to convert a dipping glaze to a brushing glaze
I have a jar of testing clear glaze that I mixed myself (10% yellow stain and 2% zircopax added to cone 03 G2931K clear). Commercial glaze producers make their lines of glazes like this. The cost of the dry materials: About $6. How can I make it paintable? I made a spreadsheet where I can specify the weight of the plastic jar, the percentage of CMC gum powder needed and the concentration of the gum solution. I just need to weigh the jar of glaze (without lid), weigh a teaspoon of the liquid glaze (lower left), dry it (upper right) and weigh the dry (lower right). After filling in these numbers the sheet tells me what weight to evaporate the jar to and how much gum solution to mix in. It paints on just like a commercial glaze. But don't do this. I made another spreadsheet online (link below) based on starting from dry ingredients, adding the correct amount of water and gum solution. Of course, you need a good mixer to do this.
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.
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.
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 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 everything will flake off). The solution? The centre piece has the same recipe but uses 85% pure raw Alberta Slip, yet it sports no cracks. How is this possible? 1% added CMC Gum (via a gum solution)! This is magic, but there is more. It is double-layered! Plus very thick strokes of a commercial brushing glaze have been applied. No cracks. CMC is the secret of dipping-glazes for multi-layering. The down side: 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?
Six layers of any normal dipping glaze would be impossible, flaking usually starts with the second layer. Yet this one is 85% clay, it shrinks so much that it would be like a "dried up lake bed" on the first layer. How was it possible to dip it on in six layers here? 1% CMC gum (via a gum solution). This is incredible! Typical dipping glazes contain only about 20% clay (plus things like feldspar, frit, dolomite, calcium carbonate and silica) and they dry within seconds and work well for single-layering. But for multi-layering these it is also vital that gum be added. There is a down side: A drying period is needed between each layer, the length depends on the porosity and wall thickness of the ware. One more thing: VeeGum is not a substitute for CMC, it is a highly plastic clay that will make flaking even worse.
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. Gummed glazes can do this, they will adhere and dry without cracking. And dry hard and resist washing off. Brush strokes hold their character. The brown glaze 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. It's brush stroke has flowed flat and it is running downward. Is it because of the lower SG? No. Commercial glazes with an SG down to 1.3 perform well also. The secret: Gum needs particle surface area to work its magic. We can get that with a bentonite addition. The dried strokes on the right were much better, that glaze adds 2% bentonite (and we raised the SG to 1.6). That made all the difference, it painted beautifully.
Powdery glaze needed gum solution. How?
I weighed the bucket of glaze and then put about a third of it on a clean plaster surface to dewater. Then I put that back in the bucket and weighed it again. It was 400 grams lighter so I added 400 grams of Laguna Gum Solution. That changed it to a drip-drip-drip dipping glaze but it now dries much harder and I can do thick brushwork on it without worrying about it lifting off.
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.
|Glaze Suspender||Gum can act as a suspending agent by virtue of the fact that it thickens the slurry.|
By Tony Hansen
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