|Monthly Tech-Tip |
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.
These lumps do not break down easily in a dry mixer, even when with other materials (like silica and feldspar). And they just bounce around on a vibrating screen. That means that without some sort of finishing device in the dry material feed stream is needed to break down these lumps before the pugmill.
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!
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.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.
The fine mineral, oxide and clay particles used in ceramics often aglommerate during storage or even in the latter stages of production. These must be broken down later.