|Monthly Tech-Tip |
Alternate Names: Barium Carb, Witherite
Barium carbonate powder is dense and white and is manufactured either from the mineral barite (BaSO4) or from barium chloride. Subsequently a precipitation process is used to get the carbonate form. There are several crystalline forms of BaCO3, alpha is the most stable.
Barium carbonate is very stable thermally and does not readily disassociate unless at least some CO is available in the kiln atmosphere (i.e. reduction). BaCO3 is reduced to the unstable BaCO2 in the reaction:
BaCO3 + CO -> BaCO2 + CO2
While BaCO2 has a high melting temperature, it will break down much readily in a glaze melt (liberating the BaO for glass building). It decomposes even more readily during glaze melting in a reduction atmospheres. The dissolution process happens most quickly if BaCO3 is present in small amounts (e.g. 5% or less). Even if present in larger amounts, the glaze matrix can solidify with both types, one participating in the glass microstructure and the other acting as a refractory filler, opacifier and matting agent (especially in low temperature glazes). Effects produced when baria is acting as a filler are sometimes mistaken for those of a true baria crystal matte. Such will likely leach toxic BaO (other oxides will opacify or produce a low fire matte i.e. CaO, MgO, Alumina, Zircon).
Barium carbonate produces gases as it decomposes and these can sometimes cause many pinholes or blisters in glazes. There are barium frits available (e.g. Fusion F-403 has 35% BaO), incorporating one of them to source it instead is a classic application of glaze chemistry calculations. The resultant glaze will be more fusible and will have better clarity and fewer defects.
In art ceramics barium carbonate is popular for the production of classic barium crystal mattes, BaO readily forms crystalline phases during cooling. These are dependent on adequate kiln temperatures, cooling cycle and the chemistry of the host glaze (a slightly reducing atmosphere is also beneficial). Some have observed that in some formulations, barium crystallizes so well that it will occur even with very rapid cooling. Barium can act to initiate crystal development in other chemistries, for example metallic glazes can benefit by the addition of some barium carbonate.
BaO can often be sourced from a barium frit (like Ferro CC-257) instead of raw barium carbonate, provided that the percentage is not too high. Do not assume that BaO is necessary in every glaze in which it appears. In some recipes BaO can be substituted for SrO or CaO and even MgO (using glaze chemistry software of course) without losing the color or surface (bright glossy blues, for example).
Barium carbonate is commonly added to clay bodies in small amounts (0.2-0.8%) to halt fired surface scumming or efflorescence It is slightly soluble in water and provides Ba++ ions to link with SO4-- ions in the water to form BaSO4 (barium sulfate). This new sulfate molecular form is much less soluble (2-3 mg/L), so it stays internal (rather than migrating to the surface during drying). However companies try minimize the use of barium (or even high clays with high soluble salts) because the barium sulphate generates sulphuric acid during firing and it corrodes kiln refractories. To get the best dissolution it is best to add the barium to the water first and mix as long as possible, then either add the water to the other dry ingredients (for plastic bodies) or add the other ingredients to the water (for slips).
See Metallurgical and Materials Transactions Volume 42B, August 2011. Page 901 for an article exploring Ba-Al-Si systems at higher temperatures.
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.
Two bisqued terracotta mugs. The clay on the right has 0.35% added barium carbonate (it precipitated the natural soluble salts dissolved in the clay and prevented 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.
|Materials||Ferro Frit CC-257|
Document of using Barium Carbonate in Brick to prevent efflorescence
Article on substituting BaO for SrO, CaO, MgO
Barium carbonate at Wikipedia
Information on baruim manufacturing methods
A Broken Glaze Meets Insight-Live and a Magic Material
Use Insight-Live.com to do major surgery on a feldspar saturated cone 10R glaze recipe with multiple issues: blistering, pinholing, crazing, settling, dusting and possibly leaching!
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
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.
Soluble sulfates in clay produce efflorescence, an unsightly scum that mars the fired surface of structural and functional ceramic products.
|Temperatures||Decomposition of Barium Carbonate (1025C+-)|
|Temperatures||Barium carbonate melts (1360C-)|
|Oxides||BaO - Barium Oxide, Baria|
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.
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.
The Use of Barium in Clay Bodies
Considerations regarding the use of barium carbonate in pottery and structural clay bodies for precipitation of soluble salts.
BARIUM and COMPOUNDS / Toxicology
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
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.
|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.|