|Monthly Tech-Tip |
The glaze in this jar was 'goop', impossible to paint on because it was too viscous. And it dried way too fast. Laguna mentions adding water so I measured the specific gravity (SG): 1.7. That is super-high, it took a 125cc addition to bring it down to 1.5, but it was still thick, dried even faster and brushing it on evenly was even harder. It was not obvious what to do next. It needed a lot more water (1.3-1.35 SG is normal to support multi-layer application), adding CMC gum and enough water to do that would produce an unusable watery and sticky slurry. Veegum CER to the rescue! It is a 50:50 mix of CMC gum and Veegum T. The former slows drying and hardens, the latter gels. So it can simply be added until the painting properties are right. And, a Veegum CER solution is easier to handle than one of CMC gum. This really worked! The brushing properties are just right and it gels nicely on standing. CER is also good for highly fritted dipping glazes or others lacking in clay content (otherwise CMC might still be better).
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).
In ceramics, the specific gravity of casting slurries and glazes tells us their water-to-solids. Body slurries especially require tight control of this property for performance reasons.
Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. There are issues compared to dipping glaze. You can make your own.
Base-Coat Dipping Glaze
These are ceramic glazes intended for dipping but which contain a gum to enable them to adhere to the body better and tolerate over-layers without danger of flaking or cracking.