Glass composition can be controlled by analyzing each new shipment of raw materials and its respective glass chemistry, requiring an analytical laboratory at each glass plant A far better method of composition control is to monitor certain easily measured physical properties of the glass that are influenced by the glass’ composition.
There are Two Composition Control Methods:
Density is defined as weight per unit volume. It is measured by comparison to a known standard using a sink-float technique, which can determine the density to the nearest 0.0002 grams per cubic centimeter.
The softening point for composition control is defined as the temperature at which a fiber of specified length and diameter will elongate at a rate of 1mm per minute, or the temperature at which the log of the viscosity equals 7.65. Using a fiber elongation technique, the softening point can readily be determined to the nearest 0.2 C.
Each oxide in the glass affects the Softening Point and Density differently:
By measuring one of the physical properties daily, a change in glass composition can be detected by a change in that property. Since density is the easiest to measure and also the most accurate, this property is measured every day at all plants. Softening point is measured at certain plants as a second precautionary measure for ensuring composition stability.
By using density as the measured property, glass composition can be controlled by Statistical Process Control techniques. By using such techniques it is possible to limit glass composition variations within detectable limits of wet chemical analytical techniques.