Complex sentence: First, the gilded surface is rubbed with a scratch brush of brass wire, until its surface be smooth; then it is covered over with a composition called gilding wax, and again exposed to the fire until the wax is burnt off.
Split into simpler sentences: First, the gilded surface is rubbed with a scratch brush of brass wire, until its surface is smooth. It is then covered with "gilding wax", and again exposed to fire until the wax is burnt off.