|Monthly Tech-Tip |
Alternate Names: Strontianite
Strontium Carbonate is a very slightly soluble source of SrO used in ceramic glazes.
Strontium is considered a safe material. Some people confuse SrO with Strontium 90, an isotope released from atomic reactions; they are not the same thing. The raw powder is low-dusting and pleasant to work with. There is disagreement about when it decomposes (data sheets vary from 1075-1100C, one even indicates 1340C) as follows:
SrCO3 -> SrO + CO2
The 'Ceramic Industry Materials Handbook' states that it starts to disassociate as early as 800C in a purely oxidizing atmosphere, whereas a CO2 atmosphere might delay break-down until around 1220C. This information is supported by the fact that when the more stable calcium and barium carbonate are added to bodies, pinholing and blistering are greater than bodies containing strontium. Wikipedia says it melts and decomposes at the same time, 1100C.
Strontium carbonate is often recommended as a substitute for barium to produce matte glazes. Use about 75% as much and test first to make sure color response is the same. However, strontium is not a substitute for barium as a precipitator of soluble salts in clay bodies because it combines with SO4-- ions in the water to form a compound that is not nearly as insoluble as BaSO4.
Viscous zirconium silicate glazes can be smoothed with the addition of strontium carbonate.
As noted, strontium carbonate produces gases as it decomposes and these can cause pinholes or blisters in glazes (if they are being generated into a glaze melt having a viscosity and surface tension that is unable to pass them or heal over properly as they escape or one that simply does not have time because of quick cooling). There are strontium frits available (e.g. Fusion F-581 has 39% SrO) and incorporating one of them to source it instead of raw strontium carbonate (a classic job for glaze chemistry calculations). The resultant glaze will be more fusible and will have better clarity and fewer defects. Strontium frits have become much more common of late and are useful to producing brilliant transparent glazes without having a high expansion (like those based on Na2O and K2O). Please read the information about the oxide SrO to fully appreciate the value of SrO (especially when sourced from frits), not just as alternatives to this material, but as excellent ways to produce brilliant glazes.
Some pottery glazes have such high strontium carbonate percentages that a frit cannot supply the needed SrO. These are, of course, special-purpose formulations, having an SrO content that exceeds (or far exceeds) normal target or limit formulas. According to the Wikipedia article, while it does have a low solubility, this increases dramatically with temperature (about 50 times greater at 100C versus room temperature) and up to 100 times greater in the presence of CO2 in the slurry. Both of these factors play in glaze slurries (they can contain other carbonates that are disassociating) and thus indicate the potential to flocculate or deflocculate them.
These are pure samples (with 2% binder added) of (top left to bottom right) strontium carbonate, nepheline syenite, cobalt carbonate, manganese dioxide, bentonite (in bowl), 6 Tile kaolin, New Zealand kaolin and copper carbonate. I am firing them at 50F increments from 1500F and weighing to calculate loss on ignition for each. I want to find out at what temperature they are gassing (and potentially bubble-disrupting the glaze they are in or under). Notice how the copper is fuming and spitting black specks on the shelf, this happens right around 1500F. These stains on the shelf darkened considerably when the kiln was fired higher.
This is a cone 10 glossy glaze. It has the chemistry that suggests it should be crystal clear and smooth. But there are multiple issues with the materials supplying that chemistry: Strontium carbonate, talc and calcium carbonate. They produce gases as they decompose, if that gas needs to come out at the wrong time it turns the glaze into a Swiss cheeze of micro-bubbles. A study to isolate which of these three materials is the problem might make it possible to adjust the firing to accommodate it. But probably not. The most obvious solution is to just use non-gassing sources of MgO, SrO and CaO (which will require some calculation). There is a good reason to do this: The glaze contains some boron frit, that is likely kick-starting melting much earlier than a standard raw-material-only cone 10 glazes. That fluid melt may not only be trapping gases from the body but creating a perfect environment to trap all the bubbles coming out of those carbonates and talc. The Aero chocolate bar of glazes!
These materials have many issues. They can create problems in glaze slurries (like precipitates, higher drying shrinkage), cause issues with laydown density and produce fired surface defects (like pinholes, blisters, orange peeling, crystallization). Lithium and barium carbonates have toxicity issues and the carbon burns off during firing (with lithium, for example, 60% of its weight is lost). Yet the oxides that these materials supply to the glaze melt - ZnO, Li2O, BaO and SrO can be sourced from frits (removing most of the problems and imparting 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 such are common in most glazes). Using glaze calculation you can often duplicate the chemistry of glazes while sourcing these oxides from frits. This being said, using the frits is about achieving a quality and avoid defects over concerns about their extra cost. Often the benefits lower the overall cost of production.
|Temperatures||Strontium carbonate decomposition (800-1100)|
|Temperatures||Strontium carbonate melts (1100-)|
Strontium Carbonate Toxicity Note
Is there any radiation danger from using this material in ceramics?
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.
Strontium carbonate at Wikipedia
|Oxides||SrO - Strontium Oxide, Strontia|
|Bulk Density lbs/cu. ft. (Packed)||105|
|Frit Softening Point||723C D|
|Density (Specific Gravity)||3.6|
|Density (Specific Gravity)||4.00|
|By Tony Hansen|
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