Steam engine gage glasses have prisms to improve water visibility.
J. David Conrad
Q What type of glass (or other material) was used for the sight glass on a steam locomotive boiler? Was it a special material, or just very thick?
— William Johnson, Eagle, Mich.
A Most (but not all) gage glasses on steam locomotive are the “reflex” type. That is, they have prisms cut into the water side of the glass so that the water level will be indicated in an easily seen line, with the area above the water level showing up clear and the area under water showing up black.
— William Johnson, Eagle, Mich.
A Most (but not all) gage glasses on steam locomotive are the “reflex” type. That is, they have prisms cut into the water side of the glass so that the water level will be indicated in an easily seen line, with the area above the water level showing up clear and the area under water showing up black.
Gage glasses are indeed thick glass, but the glass is “tempered borosilicate” for high strength and resistance to thermal shock (think: Pyrex) and also for resistance to erosion (because the high pressure water wears away the glass when the water glass is blown down).
— J. David Conrad, vice president and chief mechanical officer, The Valley Railroad Co.
