What is a “Straw Mark” or “Shear Line” in Pattern glass?
“Straw marks” are seen as irregular lines in pattern glass which may appear to be cracks and can usually be felt by running a fingernail over them.
There are many stories about how they occur such as dirt or hair in a mould or using a piece of straw to test the curing of the molten glass in the mould.
The correct name for these lines is “Shear Lines”. When a glob of molten glass is gathered to drop into a mould, the unwanted amount is cut away with shears. The cut line cools at a different rate and, although it is flattened during the pressing action, it often remains as a small line often in the middle of a piece.
Is it a defect? While not pleasing to look at depending on its size and position, it is part of the pressing process and not a defect. Will it cause the piece to crack? The sample below is on a piece of glass that is over 120 years old. So, possible but not yet.
You can see shear lines in both clear and coloured pattern glass. The latter usually less evident.
Shear lines are not to be confused with mould marks which are straight and follow the contours of a piece. They occur from the various parts of the cast iron mould.