The dreaded mirror tax, like the closet tax, the second story tax, and other mythological excise taxes never existed. The reason large mirrors were made in two or more pieces was because it was extremely difficult to manufacture large, flat pieces of glass, and even harder to transport them without breaking, and therefore they were much more expensive.
The legend may have its origin in the Townshend Revenue Acts of 1767 (left), which mandated duties on certain imported items coming from England to the American colonies, including glass. “For every hundred weight avoirdupois of crown, plate, flint, and white glass, four shillings and eight pence,” it reads. The term “plate glass” refers to a thin, polished glass containing few impurities that was used for both mirrors and large windows.
But after vigorous protest, Parliament repealed the Townshend duties in 1770 (famously, all except the one on tea), so any duties on glass were short-lived and never collected. There was no excise tax in the thirteen colonies on mirrors.