Strontium/Barium Dave Finkelnburg

Instructions

Artists who create their own glazes sometimes struggle with the questions, “Strontium or barium, which should I use? And why?” This article will help you answer those questions.

Chemical Investigation

Both chemically, and in their effects on glazes, strontium and barium are closely related to calcium and magnesium, the most common alkaline earth elements in glazes. However, each of these fluxes have different effects on glaze melting, crystal formation, glaze fit, and the response of colorant oxides. For use in glazes, alkaline earth fluxes are all available as carbonates. Calcium carbonate, often called whiting, is made by grinding ordinary limestone to a very fine powder. Magnesium is also found in limestone, notably as the variable mineral dolomite, theoretically 54% calcium carbonate and 46% magnesium carbonate. Both limestone and dolomite are abundant and inexpensive. Strontium and barium, on the other hand, occur most commonly as sulfate minerals and are converted to carbonates by expensive chemical processes. That’s why they are much more costly. Strontium and barium both brighten glossy glazes, barium even more than strontium. In both cases the brightness is a result of the effect of strontium and barium on the refractive index of a glaze, the angle at which light is reflected by the surface of the fired glaze. All carbonate glaze ingredients decompose on heating and give off carbon dioxide. Thermal decomposition of barium and strontium carbonates in glazes is reported to occur at relatively high temperatures (around cone 02). This is in stark contrast to magnesium carbonate, which decomposes well below red heat, and calcium carbonate, which decomposes completely by cone 010. The higher thermal decomposition temperatures of strontium and barium carbonates occur because of the high bond strengths between the flux elements, carbon, and oxygen. Strontium and barium have similar effects on the coefficient of thermal expansion (CTE) of glossy glazes—they reduce the CTE. When they are used in small amounts in glossy glazes, the effect on the CTE will be minimal. These fluxes are sometimes used in large amounts in matte glazes, which makes predicting the resulting CTE difficult. That’s because the glass phase and crystal phase typically have different linear CTE values. Because the amount of crystal phase formed is a result of glaze chemistry but is also influenced by cooling, only testing will reveal the CTE produced with a particular combination of glaze recipe and firing. Strontium, compared to calcium, somewhat lowers the temperature at which a glaze melts. Barium, on the other hand, somewhat raises the glaze melting temperature in comparison to calcium.

The most important thing to know about these fluxes is that strontium carbonate is not radioactive (that’s strontium 90) and barium carbonate is an effective rat poison. Yes, this glaze ingredient—a fine white powder—is dangerously toxic to people! Using barium carbonate safely requires taking proper precautions to avoid inhaling its dust. It also goes without saying that it must be kept out of the reach of children and pets, either as the powder or in the mixed glaze, as well as away from any area where it might be accidentally added to food or drink. Confusion about barium’s toxicity can occur because barium sulfate, an acid insoluble compound, is used internally in humans in medical applications. At least some barium has been reported to be leachable from most if not all glazes.

Strontium or Barium?

Why use barium in the studio? It can contribute to remarkable copper blues, beautiful satin matte surfaces, brighter glossy glazes and somewhat lower glaze expansion compared to calcium. Where white surface scumming is a problem in low-fire clay bodies, the addition of a small amount of barium carbonate can prevent this fault. The scumming is caused by gypsum (calcium sulfate) in the body. Why use strontium in the studio? It can be used to make exceptional satin to matte glazes, also lowers glaze expansion compared to calcium, and brightens glossy glazes. Are there disadvantages to using barium or strontium? Safety is certainly a problem with barium carbonate as a raw material, and barium leaching from glazes is widely reported. Both fluxes are relatively expensive. Barium carbonate decomposes at a relatively high temperature. Where pinholing and blistering are problematic a barium frit may be a better way to introduce this flux. Strontium is not toxic in any form so it has none of the safety issues associated with barium. Strontium carbonate decomposition, though, can occur at a high enough temperature to cause pinholing in mid- to high-fire glazes. Thus, the lower the firing temperature, the more likely a strontium frit should be used instead of strontium carbonate. At the least, a relatively slow firing rate near the peak temperature may be necessary to smooth glaze flaws. Because both barium and strontium are available in frits, a form suitable for use at much lower firing temperatures, excellent low-fire barium or strontium matte glazes have become possible. For the same reason, low-fire glossy glazes can be brightened with a small addition of strontium or barium by means of a frit. A common starting point for fluxes in glossy glazes is, in flux unity, to use 0.7 moles of alkalis (70% of the flux atoms) and 0.3 moles of the alkaline earth element calcium (30% of the flux atoms). If the proportion of calcium is increased, or if more than half the calcium is replaced by magnesium, barium, or strontium, satin to matte glaze finishes can be produced. This may appear simpler than it is. A glaze containing several alkaline earth fluxes can be less likely to produce the tiny crystals that form satin and matte surfaces than a glaze with only one. Barium tends to produce very small crystals, thus beautiful satin glazes, followed next by strontium, with calcium likely to produce more matte surfaces. Of course, the kiln cooling cycle is a variable that must also be considered. Formation of even tiny crystals from the glaze melt takes time. Matte glazes thus require a somewhat slower cooling cycle than glossy glazes. When using different fluxes, the fired surface is not the only glaze feature affected. Glaze expansion, color response, and glaze melting temperature may all change, too. Only testing will show whether the combination of glaze recipe, application, body, and firing will produce desired results. So what’s the bottom line, strontium or barium? Simply put, use these fluxes for specific purposes and where appropriate to achieve specific effects when no available alternatives will serve as well. Keep in mind that there are potential problems with each, but before you give up on a glaze that is great except for one or two flaws, work out a troubleshooting plan to see if you can correct them.