Float glass (a.k.a. window glass and soda-lime glass) has flat, parallel surfaces by the nature of its manufacturing process. The upper surface of the ribbon commonly called the "air" side, is fire polished. The under side of the ribbon, called the "tin" side, usually can be determined by lighting the glass surface with a special ultraviolet lamp. Under UV light, the tin side has a front surface illumination or reflection. If the tin film on the surface is objectionable, it can be removed by an additional polishing process.
There are other terms of art applied to soda lime glass and they basically indicate how the soda-lime sheets have been made, e.g., plate glass,drawn soda-lime glass, rolled sheet glass, sheet glass and blown sheet glass. Plate glass (historically known as window glass) was the name applied to glass sheets that were mechanically ground and polished. Float glass has replaced plate glass as a standard window product in the United States. Plate glass can still be purchased as a custom made product. Drawn soda-lime glass comes from an old process that was used to make thin soda-lime sheets and the process has been adapted to make micro-sheet in today's market. The rolled glass sheet process is still used to make pattern glass with the faces being fire polished. Sheet glass continues to be used and comes in thicknesses of 1/8" or less. The blown glass process originated in the middle ages and is still used to make decorative glass for stained glass windows.
The glass surfaces can be altered by sandblasting, acid etching, or by being mechanically ground to provide obscurity, light diffusion or decorative effects. Sandblasting produces a relatively rough or "ground" surface which tends to retain dirt and dust particles. Acid etching produces a finer surface than sandblasting but acid etching may become less readily available as environmental laws become more restrictive. A mechanical ground surface provides a more uniform surface than one that has been sandblasted. The ground surface can be provided in a texture from a rough to a satin finish based on the particle size of the grinding grit.
Most of these glasses can be tempered (heat or chemical tempered) to increase their strength and heat resistance.
The table below reflects the standard thickness sizes and dimensional tolerances that have been established for commerical applications. More precise thicknesses and tolerances can be held through custom fabrication. These include dimensional tolerances held within ten-thousandth of an inch, surfaces meeting optical and military standards, parallellism in measured in arc seconds and surface flatness measured by the use of Newton interference bands and interferometers. The material can also be graded to meet specific bubble and scratch/dig standards.
Although float glass has a commerical thickness range of only .040 to 1.250 inches, custom fabrication can produce parts measuring several inches in thickness down to thicknesses measured in thousandths of an inch.
The following table is derived from ASTM-C 1032, Table 2, and is utilized by the commerical sector to define the dimensional tolerances for transparent flat glass cut into rectangular shapes.
|DOUBLE (1/8 IN.) STRENGTH||0.120||0.115||0.134||3.0||2.92||3.40|