Barium carbonate has long been used as an ingredient in high-fire glazes, sometimes conferring unique properties upon glazes. One of the alkaline earth carbonates, it has also been used as rat poison (large doses can be toxic to humans as well). Glazes containing it ought to be checked for barium leaching if they are intended to hold food or drink, or reserved for surfaces that do not come into contact with food. It is not my intent to present the research on barium toxicity here, but to present a course of action for replacing it in glazes.
Li2O×Al2O3×8SiO2—lithium feldspar—HT alkaline flux—good for reducing thermal expansion, increasing thermal-shock resistance. Toxic in inhalation. Source: Clay: A Studio Handbook
SrCO3 Alkaline earth, high-temperature flux, similar to barium, slightly more powerful.
Li2O×Al2O3×4SiO2—lithium feldspar—powerful high-temperature alkaline flux. Promotes copper blues—good for thermal-shock bodies and matching glaze. Toxic in inhalation. Source: Clay: A Studio Handbook
CaO×3B2O3—traditional important LT alkaline flux, but is no longer being mined. Replace with Ferro 3134 for LT glazes, commercial Gerstley borate substitutes for HT glazes. Test all substitutes. Source: Clay: A Studio Handbook
Na2O×2B2O3×10H2O—a major LT alkaline flux, available in granular or powdered form.
Basic, opposite of acidic—chemical nature of many fluxes. Source: Clay: A Studio Handbook