Replacing glass is a part of life for those who use exposure units. Coudray examines the ways in which various types of glass affect exposure efficiency.
Common glass used for windows and architecture is made up of silica (sand) soda ash, limestone, dolomite, feld¬spar, sodium sulfate, and cullet (recycled broken glass). The percentages vary depending on the use and the manufacturer. Trace oxides of metals often impart color or are added to tint the glass. Small defects can appear inside the glass during its manufacture. These are called inclusions and are generally small bubbles. Surface defects are called seeds and appear as small bumps on the surface of the glass. For our purposes, the glass must be free of both types of defect. You need to specify that you want seed-free and inclusion-free glass. If the glass is custom cut (it always will be) and you have not specified this, you will end up with glass you can’t use.
There are other types of glass like borosilicate (Pyrex) and synthetic quartz. These fall into a category known as technical glass and are generally used for scientific glassblowing, optical components like lenses and filters, and glass labware like test tubes, beakers, and flasks. They have UV transmission of 90% or higher.
Window glass needs to be distortion-free when you look through it. If it isn’t, light will refract at a different rate and you could potentially undercut an image locally. This is not some¬thing to worry about. Modern glass is distortion-free.
The need for distortion-free glass was not a high priority until the 19th century. Widespread use of silvered mirrors and glass for windshields in cars increased the need for optically clear, distortion-free glass. The solution was plate glass, made by way of an expensive process that began with molten sheet glass passing between mechanical steel roll¬ers. Uneven rolling tended to impart some distortion to the glass, and sign-ificant quantities of the glass were ren¬dered useless because of roller marking. Sheets that passed quality inspections to this point moved to the polishing phase. The entire sheet was polished until both surfaces were parallel and the sheet itself was free of any defects. This was a very expensive and time-consum¬ing process. You can still purchase plate glass today, but it is custom produced to order and quite expensive.
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