The glass surface

1) The glass surface along the line of cut must be cleaned before the score is made [fig 3]; 2) the wheel must roll freely and be in direct contact with the glass to develop the proper fissure [fig 2].

If the wheel must cut through any contamination, stain, or dirt particles, much of the energy required to make the fissure will be lost. An irregular fissure or a skipped fissure will then be made, resulting in chipped and weak edges at break out.

Glass surfaces are usually coated with a special powder by the manufacturer. This powder protects the glass in shipment. It must be removed from the line of cut to prevent pick up by the cutting wheel. Cleaning the glass along the line of cut also removes this powder.

As stressed above, the wheel must roll freely at all times. Before attempting to score the glass be sure that the wheel slot is clean and that the wheel roils freely, without drag. It is vital that the wheel axle be lubricated.

Figure 2

Figure 3

The fissure is very shallow but can easily be seen because its sides are mirror surfaces and readily reflect light. No glass chips or glass dust should be on the surface if a proper fissure is made. If the surface is not clean and if the score is a white gritty line, this indicates that a gouge or scratch has been made, not a true fissure. 

A sheet can be broken out along a gouge or scratch: however, the edge will be chipped and weak. Such an edge should be ground or belted to remove the nicks and avoid almost certain glass failure.