muff
A blown cylinder of glass which is afterward flattened out to make a sheet.
muff: a blown glass cylinder destined for sheets
A muff is a hollow cylinder of glass formed by blowing air into molten glass, typically 4 to 6 feet long and 12 to 18 inches in diameter, which is then cut and flattened to produce window panes or other flat glass products. This was the dominant method of making sheet glass from the medieval period until the early 20th century, when mechanical processes like the Fourcault and Libbey-Owens methods replaced it.
The glassmaker gathers a blob of molten glass on a blowpipe, then blows it into a roughly cylindrical shape. The cylinder is kept in motion and reheated to prevent uneven cooling and cracking. Once the muff has cooled enough to hold its shape but remains pliable, the glassmaker uses a hot iron or flame to score a line along its length, then splits it open lengthwise. The still-warm cylinder is then laid flat on a lehr or annealing table, sometimes with weights applied, and allowed to flatten naturally as it cools.
The resulting sheet glass from a muff is uneven in thickness, typically ranging from 1/16 to 3/32 inch, with visible surface variations and occasional bubbles or stones. The edges are ragged and must be trimmed. These sheets were graded by quality: the best went to fine windows, while inferior pieces were used for less demanding applications or re-melted. The texture of muff glass, particularly its subtle waves and slight distortions, became prized in restoration work and period architecture.
Muff glass in practice
Muff production required skilled labor and was labor-intensive: one team of glassmakers might produce 100 to 150 sheets in a working day. The process was subject to many variables, particularly the skill of the blower and the composition of the glass batch. Crown glass, made by a related process where the blown cylinder was spun to form a disk rather than flattened, competed with muff glass for window applications and often commanded higher prices because it was more uniform.
The term muff survives in glassmaking vocabulary and in conservation, where specialists can often identify whether a piece of old glass was made by the muff method, crown glass process, or later mechanical rolling. For heritage restoration, sourcing historically appropriate muff glass requires either salvage from period buildings or commission from the few remaining craftspeople who still practice the technique.