Alternate Names: Barium Carb, Witherite
Two bisqued terracotta mugs. The clay on the right has 0.35% added barium carbonate (it precipitates salts dissolved in the clay to prevent them coming to the surface with the water and being left there during drying). The process is called efflorescence and is the bane of the brick industry. The one on the left is the natural clay. The unsightly appearance is fingerprints from handling the piece in the leather-hard state, the salts have concentrated in these areas (the other piece was also handled, but has very little marking).
An example of how effective barium carbonate is at precipitating the soluble salts in a terra cotta clay. These two unglazed, cone 04 fired, mugs are made from the same clay, but the one on the right has 0.35% added barium carbonate.
Low temperature clays are far more likely to have this issue. And if present, it is more likely to be unsightly. The salt-free specimens have 0.35% added barium carbonate.
These materials have many issues. They can create problems in your glaze slurries (like precipitates, higher drying shrinkage), cause issues with laydown and dried surface and cause fired surface defects (like pinholes, blisters, orange peeling, crystallization). And lithium and barium have toxicity issues (as raw materials). And the lithium, barium and strontium are carbonates, that means carbon burns off during firing (with lithium, for example, 60% of its weight is lost). Yet the oxides that these materials source to the glaze melt, ZnO, Li2O, BaO and SrO can be sourced from frits. In doing that you can solve almost all the problems and get better glaze melting. Fusion Frit F 493 has 11% LI2O, F 403 has 35% BaO, F 581 has 39% SrO and FZ 16 has 15% ZnO. Of course, these frits source other oxides (but these are common in most glazes). Using glaze chemistry you can often duplicate the chemistry of a glaze while sourcing these oxides from frits.
Soluble sulfates in clay produce efflorescence, an unsightly scum that mars the fired surface of structural and functional ceramic products.
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
|Temperatures||Decomposition of Barium Carbonate (1025C+-)|
|Temperatures||Barium carbonate melts (1360C-)|
|Minerals||Gypsum, Calcium sulphate|
Document of using Barium Carbonate in Brick to prevent efflorescence
Article on substituting BaO for SrO, CaO, MgO
Information on baruim manufacturing methods
Barium carbonate at Wikipedia
|Media||A Broken Glaze Meets Insight-Live and a Magic Material|
|Materials||Ferro Frit CC-257|
|Hazards||The Use of Barium in Clay Bodies|
|Hazards||BARIUM and COMPOUNDS / Toxicology|
|Hazards||Barium in Materials and Fired Glazes|
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Materials that source Na2O, K2O, Li2O, CaO, MgO and other fluxes but are not feldspars or frits. Remember that materials can be flux sources but also perform many other roles. For example, talc is a flux in high temperature glazes, but a matting agent in low temperatures ones. It can also be a flux, a filler and an expansion increaser in bodies.
|Articles||Soluble Salts in Minerals: Detailed Overview
There are a wide range of soluble materials that can be in clay, this article enumerates them, provides procedures on identifying and measuring them and outlines what to do about the problem.
|Oxides||BaO - Barium Oxide, Baria|
|Density (Specific Gravity)||4.27|
|Glaze Opacifier||If available in sufficient amount, barium oxide will promote crystallization of a melt during cooling, thus imparting a measure of opacity.|