Thanks Bored Chemist. I have learned so much from you
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Yes, lead oxide is definitely poisonous (and volatile!) I would highly recommend against using it.
Perhaps you could try a different approach to making synthetic corundums—growing crystals out of solution instead of from a flux or melt.
At room temperature, aluminum oxide has very high solubility in highly alkaline aqueous solution (forming aluminate ions), and can be precipitated out again with a decrease in the pH. Slowly changing the temperature and/or pH of the solution may allow growth of aluminum oxide crystals.
Hydrothermal synthesis of sapphires is a known alternative to synthesis from molten aluminum compounds.
Anything that did so would stop it being cast iron.