Shock Collar Question - July 31

Brooke is wearing the Shock Collar this morning!

Today's question: In Ancient Egypt people had to pay a tax to borrow cooking oil. In 1700 the Russian emperor put a tax on beards in a hope to get men to be clean shaven like western Europe. But in Ancient Rome, during the 1st century, the Roman emperor put a tax on a special entity that at the time was a key ingredient used to make many products. What item was it?

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