Here are the three common lithia source materials in a side-by-side calculation to compare their potencies. I have adjusted the material weights so that all are delivering the same amount of Li2O (assuming a recipe calling for 5% lithium carbonate).
By weight it takes 42 parts of petalite to deliver the same amount of Li2O as 5 parts lithium carbonate! But notice how much SiO2 and Al2O3 come along, major surgery on the host recipe is needed to compensate for this (if even possible). Spodumene is complicated too, it brings along so much Al2O3. That’s a problem because the most frequent existing Al2O3 supplier, clay, may have to be greatly reduced or even eliminated (producing a recipe that does not suspend in water). The other baggage both materials bring is lots of SiO2, especially petalite. Thus, replacing lithium carbonate with petalite or spodumene is not really a material substitution—it is a glaze reformulation.

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Of course, if a recipe only calls for 1-2% lithium carbonate, either of these might be candidates to supply the Li2O. But Petalite is 8x less concentrated and Spodumene 5x less. Not surprisingly, prices reflect this (unless availability is the issue). But when a recipe calls for 5% lithium carbonate, there is another problem: Petalite is extremely high in SiO2 and spodumene is very high in Al2O3. Introducing enough of either of them to supply the same amount of Li2O as 5% lithium carbonate can easily oversupply either. While extra SiO2 can often be accommodated because most glazes contain significant silica, bringing in a lot of Al2O3 necesitates cutting the most important normal source: Clay. The result is a slurry that does not harden and does not stay suspended.
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