Testing your Scoring Pressure

Most often people are asked to listen to the sound of their scoring.  Unfortunately, different glass styles make different sounds. Float glass makes a particular sound, transparent stained glass makes a slightly different one, and opalescent glass makes almost no sound. Consistent pressure of the right amount is important to clean, consistent breaking of glass. Therefore, we must learn to cut with the same consistent pressure on all types of glass, rather than listening for sound.

Pressure

The general recommendations for the pressure to use during scoring is 6 - 7 Lbs or 2.7 – 3.2 Kg. This is difficult to judge. I found that I needed a means of letting people know for themselves the pressure they were exerting. It is not enough to watch and say that was too hard, that was too soft, etc.

A digital kitchen scale approximately 15cm/6" square

This is my method of teaching novices how to judge the pressure they are using. I get out a digital kitchen scale that can have the scale set to zero whatever is on top of it. Place a piece of glass no larger than the platform on top of the scales. Zero the scale display.  Have someone watch the scale display while you score in your usual way. Of course, you must not touch the glass with your other hand. Have them tell you the maximum and minimum weights displayed. Keep repeating until you can consistently use that 6 - 7 pounds (2.7 - 3.2Kg) pressure.

Scoring at 2.288kg/5 lbs

Score lines for 1.8kg and 3.5kg/4 lbs and 8 lbs. you can barely see the lighter score line.
But both break well as straight lines

Straight lines scored with light, medium and heavy pressures break well.

When curves are scored heavily breakouts occur. This was scored at 4.3kg/9.5 lbs 

Consistency

The other important element of scoring is to keep the pressure consistent throughout the score. This test will also show how evenly you apply the pressure during the score. The objective of scoring is to use the correct pressure throughout the length of the score. If your pressure varies significantly during the score, it will be difficult to get the glass to break consistently along the score line. This is because the amount of weakness in the surface created by the score is variable.

Your observer can tell you when the pressure is less than optimum or more than desired.  If this has a reasonably consistent place in scoring, you can concentrate on correcting that. For example, most people start off with a lighter pressure than further into the score.  Getting the feel of the correct pressure will enable you to apply it right from the start of the score. Sometimes, people increase the scoring pressure when they come to curves. This test will show if that is true for you.

This testing can take quite a while. But it is worth the time spent in getting the scoring pressure right and reduce the number of unwanted breaks. However, it is not a one-time test. When I begin to have difficulties in breaking glass, I go back to this test to check whether I am scoring too heavily. In my scoring practice, I find that my best ones are those with 1.8 to 2.5kg (4.0 to 5.5 pounds) with the cutter I use.  This is less than many, but it has worked well for me for years.

There are, of course, other elements that go to make up a good score and break. But the most important thing in scoring and breaking opalescent glass is to avoid too heavy a score by listening for a sound. Cut to a consistent pressure whatever sound is heard.