Crushed oyster shells have been used in clay bodies as a grog, but are only useful below 1500-1600 (they may cause lime blows above this). Crushed to a fine power for use as oxide sources, in glaze melts for example, they are one of the purest forms of theoretical calcium carbonate (having lower iron and alumina especially).
The chemistry shown here is from Characterization of Sand Casting and Oyster Shells as Potential Sources of Raw Material for the Production of SodaLime Glasses in Vol 43, 2015 Chemical Engineering Transactions (http://www.aidic.it/icheap12/535maia.pdf). The CaO, SiO2, Fe2O3 and LOI percentages are in harmony with other types of sea shells (mussels, clams), MgO is higher, Al2O3 is lower. All are quite similar to limestone.
Flux Source 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.