|Monthly Tech-Tip |
An ordinary kitchen blender does a better job of mixing a ceramic slurry than any industrial machine. It even reduces clay particle size and hyper ages it.
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Anyone who has an inexpensive kitchen blender can mix glaze batches better than any industrial equipment. These machines can inject so much energy into a slurry that all agglomerates are broken down, particle surfaces are better wetted and many types of clay particles unaffected by normal mixing are actually pulverized to smaller sizes. Recipes containing gum powders can be notoriously difficult to mix with water but using a blender it can be done quickly and thoroughly. Blending a slurry (body or glaze) also quickly ages it, stabilizing the rheology.
Mixing large batches of slurry with a blender, at least for studio use, is practical, especially compared with the difficulty of sieving (which is a batching process also). It is just a matter of repeating a cycle of filling the jar, blending for 30 seconds and emptying.
Many consumer blenders are practical. It is best to use one that has a glass jar, or buy a glass jar on ebay.com. The better brands are the ones with the most active spare parts and accessories markets. Blenders having a metal-to-metal coupling between motor and propeller are the most durable. Industrial blenders with fairly large jars are also available on ebay.com.
Blender mixing is invaluable in slurry preparation in small scale ceramics and testing. It is quick and so effective that not only are particle surfaces wetted much better but clay particles can actually be reduced in size (literally ground finer). Slurry rheology is also stabilized. For example, thixotropy can often be achieved naturally, without any additives. Materials that are otherwise impossible to mix into a slurry (e.g. bentonite, Veegum, CMC gum) are no problem. Even slurry-processed porcelains benefit, not only being more plastic, but firing to a more homogeneous surface and to greater density. Just visually, it is easy to see how much improved this MNP slurry is (a local clay with porcelaneous properties). It was mixed using our propeller mixer and seemed OK (on the left). But the improvement after only 20 seconds in the blender (right) is amazing.
This brushing engobe is thick and gooey (because it contains CMC gum), so it is practically impossible to sieve. While our propeller mixer was not able to break up the tiny agglomerates of New Zealand kaolin 30 seconds of bender mixing made it as smooth as silk. To make this liter of brushing engobe we use 800g of powder and 10g of CMC gum in 800g of water. It does not require Veegum (like glazes) because it already contains plenty of clay. The CMC gum greatly improves the brushing properties so additions of stain make it useful for colored underglazes also. For best performance we apply them to leather hard ware.
Dipping glaze applies and dries in seconds. Brushing glazes dry slowly and dry hard. Brushing glazes simply have gum in the recipe, dipping glazes typically do not. This is the Alberta Slip based G3914A, a jet black gloss cone 6 glaze, it has only 4% added black ceramic stain. We normally mix this as a dipping glaze but I have made a 500ml jar of low SG brushing version using both of the following methods. These methods will work for almost any glaze recipe (for those having exceptionally high clay content less Veegum is needed). 1) Shake together, in a plastic bag, 340g of mixed glaze powder with 5g of Veegum and 5g of CMC gum. Add that to 440g of water in a kitchen blender and mix on high speed until it gels (the gums resist mixing so the highest speed for at least 30 seconds is needed to prevent lumps). 2) Take 680g of dipping glaze (assuming it is about 50:50 water:powder you get 340g of dry), put that in the blender with another 80g of water and proceed as in method 1. Less total water is being used because the dipping glaze might not be exactly 50:50 water:powder. During mixing, if it gels too much add the extra 20g of water. This is so exciting that I make fancy labels, they are just ink-jetted onto regular paper, cut 62mm wide (2 7/16") and held securely on with 2 7/8" transparent packing tape. You'll need base glazes for this, then add stains, opacifiers and variegators.
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!
In ceramic studios, labs and classrooms, a good propeller mixer is essential for mixing glaze and body slurries.
The process of slurrying a clay body powder and dewatering it on a plastic slab or table.
|By Tony Hansen|
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