Strontium Carbonate is a very slightly soluble source of SrO used in glazes.
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 colour 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.
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.