Here is an article I posted a while back, worth another look see:
Brasses are a window back in time. More prevalent than a painting, surviving textiles, or armor from the medieval period, they tell the story of all walks of life, from merchants, to jugglers, to knights and squires; a permanent representation of a society long gone.
Brasses are metal tablets engraved with the image of the one memorialized. The metal is often referred to as “latten” or “a latten” (from the French laiton, brass). According to the Dictionary of Chivalry, the plates were not even true brass “but are compounded of about 60-65% copper, 10% lead and tin, and perhaps 30% zinc. Till about 1550, latten was always imported from Flanders and Germany, often coming through Cologne; hence, they were sometimes called ‘cullen’ plates.” Don't you just love language!
The thing about brasses is not necessarily their spot-on rendering of true portraits, but in the invaluable detail of the clothing and armor of the time. Something like 8,000 brasses survive in England alone, from the 13th to the 18th centuries. Some brasses are life size while others—set in churches—are smaller representations.
Making a rubbing from these brasses (that is, laying a piece of paper over the brass and using a crayon or some other media to transfer the image onto paper) became somewhat of a mania in the 1960s. So much so, that the practice was discouraged by the government and churches in the fear that the details on the brasses would be rubbed away from so much use. That fear has relaxed and there are in fact many brass rubbing societies in Britain today. But ask before you rub (a good rule of thumb for any and all occasions, I’m sure) because some of the brasses are fragile and can be damaged by rubbing and so they aren’t allowed. And many are located in churches so it is definitely not on to do so during a church service.