Gravity

One of the fundamental elements in kiln forming is gravity. When glass is hot it moves according to the effects of gravity and you have to remember that gravity has a big effect on all your firings.

The effects mainly cause:

• Uneven thickness on shelves that are not level.
• Uneven slumps into moulds which are not level or the glass is not levelled.
• Uneven forming due to varying viscosities. Gravity acts on the softest parts of the glass first.
• Faster or slower forming due to span width. With greater span, gravity pulls the glass into the mould more quickly than with a small span.
• Gravity acts on things of greater thickness more quickly than those of lighter weight. So a thick piece will form more quickly than the same sized thin piece.
• Surface tension (affected by viscosity and heat) is affected by gravity also. The glass will attempt to draw up or spread out to about 7 mm if there is enough heat, time, and low viscosity.
• The effect of gravity causes upper pieces to thin lower ones, as it presses down while the glass is plastic. This has the effect of making the colour of the lower piece less strong.

More information on each of these effects can be found throughout this blog.

Glass Shifting on Mould

There are a number of things to investigate if your blank is shifting on the mould during firing.

Is there a heat differential?

Glass absorbs heat at different rates depending on colour and type meaning that one part may begin to move before another. The solution to this is to slow down the rate of advance to allow all the glass to gain heat at the same speed. It may also be useful to slump at a lower temperature.

There also may be a heat differential within the kiln. You need to run a check on the heat distribution of your kiln to be sure where the (relatively) hot and cold areas of your kiln are. Bullseye published Tech Note no.1 on how to do this.

Not perfectly balanced on the mould?

Glass can be placed just off square or level and that can allow it to start slumping unevenly. Measurements and observation can help to get the glass placed squarely on the mould. Also a small spirit level placed on the glass can tell you if the glass is level within the mould.

The mould may not be level.

The kiln, shelf and mould should each be checked for level in all directions. The kiln level can be established and can be assumed to be level until it is moved. The shelf level should be checked each time it is moved. The mould level should be checked each time it is used.

Is the glass overhanging the mould?

Glass overhanging the mould rim can hang up on some of the edges more than others. Check the rim of the mould for any rough areas and smooth them. If you do have glass overhanging, you should slow the rate of advance to allow the edge of the glass to tip up and begin to slide down into the mould. If the problem persists, make the glass blank smaller, or support the overhanging glass with a collar.

Is the glass heavier on one side?

The glass may be uneven thickness and so heavier on one side. The heavier area of the glass will begin to slump first and so promote movement of the whole glass in an asymmetrical manner. The solution to this is to fire slower and to a lower temperature.

Do you have a wonky mould?

The mould can be imperfect. So you need to check the mould for accuracy. I have a slumper that has one side lower than the other three. Being aware of this, I can place the glass so that it is still useable. Measuring the mould in all directions will help determine its symmetry.

If all these things have been investigated and the solution not found, it is possible to create a bevel on the bottom edge of the glass so that the edge sits in the mould at the same angle as the mould. This provides a larger contact point for the glass and mould than just a thin edge. This appears to allow the glass to move evenly during the slump.

Of course, a major solution is to observe the slump.  Peeking into the kiln at the beginning of the slump soak and frequent intervals after that will show if the piece is slumping evenly or not.  If it is uneven, you can put on the appropriate protective gear and with gloves on your hands, shift the glass to be set evenly in the mould.

The major solutions to avoid uneven slumping are:

• Avoiding the hot and cool parts of the kiln
• Making everything level
• Careful placement on the mould
• Slower rates of advance
• Lower slumping temperatures
• Observation