Fiberglass cloth is made from weaving yarn that is made from glass. The glass is melted in ovens and pulled through holes in dies that is drawn down to perfect glass rods. There are three main types of glass used in the fiberglass cloth Thayercraft, Inc sells. They are C-Glass, E-Glass and S-Glass, with E-Glass the major type we sell. There is also Quartz, or pure silica, used in high temperature fabrics that is essentially pure Silicone Dioxide. C-Glass is a Chemical resistant type of glass composition used in Stucco and concrete reinforcement for its alkali resistance. S-Glass is essentially a stronger version of E-Glass used in composites having corresponding types of weaves and similar style numbers. The most notable differences in the composition of E-Glass and S-Glass is that S glass has a higher concentration of Silicone Dioxide (65% for S-Glass compared to 55% for E-Glass), no or negligible amounts of Calcium Oxide for S-Glass where E-Glass has 16 - 25%, double the amount of Aluminum Oxide (25% vs 14%), and more Magnesium Oxide. S-Glass also has the addition of about 10% Magnesium Oxide where E-Glass is minimal. Though only small amounts are found in E-Glass, S-Glass is free of Boron Oxide, Titanium Oxide and Fluorides. S-Glass is considered to be 20% stronger than E-Glass and is more expensive, by what I've heard from techs at the yarn producer, because of higher equipment costs from the higher processing temperatures.
E-Glass, the one used in most of the weaves we sell, is designated as "Electrical" for it's lack of conductivity and is the glass used in circuit boards, electrical insuation, woven roving, all the lightweight cloth that we have under the model category, boat and tooling, cloth for kayaks. The only cloth we sell that is not E-Glass is grouped in it's own category with the style numbers of S-Glass we have being 6581, 6781, the heavy satin weave 9773 and some other S-Glass weaves that come and go. 6581 is the S-Glass version of 1581 8.9 oz/sq yd 8 Harness Satin and 6781 is the S-Glass version of 7781. We do have some C glass that is coated with PVC for stucco and concrete reinforcement.
Molten glass, of any chosen composition, is pulled through million dollar dies with holes small enough to allow multiple strands being pulled continuously and in a way to create long perfect true monofilament rods. The smaller the diameter of this glass rod the stronger the yarn thus the stronger the cloth. This can be visualized by comparing a bundle of these glass rods to the strands in a rope. The more strands in the rope the more a possible breakage is spread out. Just imagine a single 1/4" glass rod being pulled in tensile and scratchng it somewhere with a hacksaw blade allowing it to easily break. Compare this to a 1/4" bundle of glass yarn and tryng to cut it with a hacksaw. So you can see the smaller the diameter of the glass rod used in the yarn the greater the tensile strength of the yarn and resulting cloth. There has been experimentation with filament diameters down to 1 micron with incredible tensile values. Problems were found in producing filaments this small and were not practical but yarn manufacurers commonly produce strands down to 3.5 microns. Filament diameters used in the cloth we sell normally range between 4.5 microns to 13 microns. Glass filament diameters have a letter designation system as illustrated in the following table using that given in one of BGF Industries Handbooks. These are "nominal" diameter values and it's not clear if the inches was converted to microns or vise versa but this will give an idea of the filament size.