Short Holds in Schedules - Kiln Forming Myths 5

Frequent short soaks on the way up will make a schedule safer

Safer in this context usually means less subject to thermal shock.  To determine the validity of this requires a bit of understanding on how the glass takes up heat, and as effected the lay-up.

Glass is a good insulator, both of heat and electricity, although we are only concerned about heat here.  This means that glass transmits heat poorly or, as it may be thought of, slowly.  A steady input of heat at an appropriate rate is less likely to shock the glass than quick rises with (catch up) soaks. 

In general, there is not much change in the rate required when you go over to a single rate without soaks.  For example, a ramp rate of 200°C from 20°C to 400°C with a 20 min soak, then 300°C to 540°C with another 20 minute soak could also be written as 193°C/hr to 540°C - both take 2.8 hours to achieve the same temperature. So the rate is not very different, but the way the heat is put into the glass is.

The glass is subject to heat shock below its softening point, and so rapid increases in temperature at the start of the schedule increase the risk of thermal shock below the 540C region.

When you have uneven coverage of the base glass, as most of us do, more care is required than when we have evenly thick glass.  This relates to the poor heat conductivity of glass.  The need is to have all the glass heat up at the same rate.  This is relatively simple when there are no partial layers on top as when doing a decorative tack fusing.  The pieces on top insulate the heat from the glass immediately below.  This gives a cool spot under the top glass, in relation the uncovered glass. To avoid this difference in temperature, which causes stress, becoming too great you need to slow the rate of advance as well as keeping it a steady increase.  This indicates you should be scheduling the rate of increase as though there were two more layers over the base glass.

The steady input of heat also becomes more important with thicker glass or more than two layers of glass.  The rate of heat input needs to decrease rapidly with increasing thickness – there is not a linear relationship.  For example, doubling the thickness from 6 to 12mm requires a reduction of 2.3 times the rate of advance.  Increasing the thickness by 4 times to 25mm requires a reduction of 10 times the 6mm rate of advance.

Other factors that require slower and steady increases in temperature are where you have dark and light glasses next to one another.  The same applies where you have a viscous and a less viscous glass together.  The classic is black, the least viscous of the glasses, and white, the most viscous.

However there is at least one circumstance where soaks are useful.  When draping over steel or ceramic, the free hanging glass heats up more than the centre where it is resting on the mould.  In this case, the mould forms a heat sink, drawing the heat away from the glass into itself.  You need to go very slowly or insert a few soaks to allow the supporting mould to heat up. More information here.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.