Wednesday, 2 March 2016

The Effect of Glass Temperature on Cutting

There are many opinions on how glass cuts when cold.  Some report cutting outdoors in sub-freezing temperatures, others that only warm glass cuts well.  I decided to see what scientific information there may be on this idea.

The Science
The scientific literature mostly concentrates on the effects at higher temperatures than we are concerned with.  However, there are some things that are applicable, and some of these effects of temperature are outlined below.

·         High humidity results in loss of strength. 

·         The strength of glass is reduced by 25% at 100°C compared to 0°C.

·         Glass needs several days to be at an even temperature throughout.

·         Variance in temperature across the glass causes unwanted breakages.

·         Colder glass becomes more brittle due to loss of elasticity.

·         Hardness of glass increases with decreasing temperature.

The terms of strength, hardness and brittleness have scientific definitions that are hard to apply to the everyday glass cutting that we do.  Strength may or may not have applicability to glass cutting.  Elasticity may or may not be an important factor in cutting.  Surface hardness may play a part in cutting while cold.

Applicability of the Science
However some things seem to apply. 

High humidity results in loss of strength.  This may be a factor in low temperature cutting.  The humidity in a relatively closed environment increases with the reduction in temperature.  Breaking glass is about the creation of a weakness in the glass along the score line.  In so far as strength is a factor in the break running along the score line, this may be an element in cold glass cutting.  If the whole glass is weaker, the difference in strength at the score line is less and so promotes unwanted breaks.

Variance of the temperature of the glass throughout the substance of the glass promotes unwanted breakages.  Perhaps the cold glass that is difficult to cut is not equally cold throughout.  Certainly a number of people report that they store their large glass outdoors and can still score and break the glass during the winter perfectly well before bringing it into the studio. 

Glass becomes more brittle with decreasing temperature, and it also becomes harder.  Perhaps these two elements are a factor in controlling breakages.  If the glass is both harder and more brittle, a different scoring method is required. 

The way in which glass at any temperature breaks is related to the force of the score, the speed of the score and the angle of the cutting wheel.  If the glass is both harder (at the surface) and more brittle it requires less scoring force or a blunter wheel angle.  The more blunt the wheel on a thicker (i.e. stronger) glass, the more vertical the stress lines are created in the glass.  So in a cold and harder glass, a blunter wheel angle seems appropriate, even though the glass is not thicker.

It is not usual for people to have cutting wheels of different angles, so an easier, although more skilled, approach is to reduce the scoring force in cold conditions.  Reducing the force in scoring a hard and brittle glass causes the stress lines to be more vertical than increased forces do.  Increased forces cause lateral lines of stress to be created, leading to unwanted breakages.

Secondly, the glass being more brittle, less force in breaking stress is required.  As the glass becomes colder, the less elastic it is.  This elasticity is an important element in breaking the glass at room temperatures. The score needs to be run gently to counteract the loss of elasticity and the consequent increase in the brittle strength of the glass.

My conclusion, after the reading I’ve done, is that cold glass becomes slightly stronger and more brittle than room temperature glass, and so requires a slightly different method of cutting. This difference is to reduce the pressure of scoring and the force of breaking (applying stress to the glass).  

Of course you can warm the glass up before scoring it, but the research seems to indicate that significantly long times are required to equalise the temperature throughout.