Alternate Names: Barium Carb, Witherite
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|DENS - Density (Specific Gravity)||4.27|
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.
Glaze Opacifier - White
If available in sufficient amount, barium oxide will promote crystallization of a melt during cooling, thus imparting a measure of opacity.
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.
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.
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).
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.
Out Bound Links
BARIUM and COMPOUNDS / Toxicology - Edouard Bastarache
Barite (65.7% BaO, 34.3% SO4) is the main mineral from which barium carbonate is sourced. It is mined around the world and generally occurs in beds or as veins. It is found associated with lead ore, c...
Overview of strategies in dealing with the hazards of difficult-to-replace barium carbonate in ceramics
The mechanisms of decomposition for BaCO3, CaCO3 and SrCO3 are similar (although of course not at the same temperature; calcium carbonate, for example...
Barium carbonate mineral.
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.
This is an outline transcript of the video. Use the link below to go to the video itself. The video is about this cone 10R rutile blue glaze (see close-up photo below) and how to fix its problems u...
In Bound Links
Hazards of barium carbonate, considerations regarding its use in clay bodies for precipitation of soluble salts
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
Sodium, potassium, magnesium sulfates can be found in many clays. These are soluble and migrate to the surface during drying. Soluble salts can be iron stained, but often are white. Depending on the temperature of the firing and the nature of the salt, they can leave scum on the surface (often calle...
CaSO4 2H2O is the crystalline mineral from which plaster is made. It is not practical as a source of CaO in glazes because its decomposition produces SO3 which is dangerous to health and it is destruc...
A term describing the whitish or brownish dry or glassy scum (depending on iron content and firing temperature) left on the surface of a fired clay body (most often red earthenware or raw stoneware and fireclays). Many clays contain soluble sulphates that migrate to the surface with the water and ar...
A glaze that is not glossy. Of course, unmelted glazes will not be glossy, but to be a true matte a glaze must be melted and still not glossy. To be a functional matte it must also resist cultery marking, clean well and not leach into food and drink. Thus it is not easy to make a good matte glaze. I...