The desire is to have the same degree of fusing on both sides of the glass. An example is where a person wants to have their colourline paints equally matured on both sides of the glass in one firing. This is difficult and requires a different strategy than normal fusing.
A bit of background first. Glass is a very good insulator. This means that heat travels slowly through the glass. Its practical effect is that we have wavy lines on the top and very crisp lines on the bottom. This results from the temperature differential between the two surfaces. This can be many degrees different during the plastic phase of the glass. It is dependent on how fast the temperature rise is. The faster the rise in temperature, the greater the difference as the glass transmits the heat from top to bottom so slowly. The problem is how to keep the temperature differential as small as possible.
The concept of heat work relates to the way heat is put into the glass. It can be done quickly to a high temperature, or slowly to a low temperature and still get the same effect. This shows glass reacts to the combination of temperature and time. Putting heat into the glass slowly allows lower temperatures to be used to achieve the desired effect, than fast rises in temperature.
The insulating properties of glass means that the heat work needs to be applied slowly to achieve similar temperatures on both sides of the glass. The thicker the glass the longer it will take to temperature equalisation.
The mass of materials also needs to be considered. The glass will normally be on a ceramic shelf of 15mm to 19mm. This mass also needs to heat up to the temperature of the top of the glass. Until it does, it will draw heat from the glass. This also points to the need for slow heat input.
The question that prompted this note was how to get glass strainers paints to have the same degree of maturation on both sides at the same time. The maturation temperature of Reusche tracing paints is around 650°C. If you use a normal rate of advance – say, 200°C – the bottom of the glass will be considerably cooler than the top. This is both because of the insulating properties of the glass and the mass of the shelf.
Methods to achieve the effect.
Some methods are worthy of consideration separately or in combination.
Use refractory fibre board as shelf. This dramatically reduces the mass of the shelf to be heated up. This kind of shelf requires more care to avoid damage than a ceramic shelf. It would be possible to place smaller fibre shelves on top of the standard ceramic shelf rather than having one large fibre board shelf. This will not be so efficient an insulating mass as fibre board on its own. Also, it will not be sufficient on its own to obtain equal temperatures on both sides of the glass.
Use 3-6mm refractory fibre paper between shelf and glass. This again reduces the heat sink effect of the ceramic shelf, but not as much as a fibre shelf on its own. Again, the fibre paper on its own is not enough. The scheduling is important.
Use very slow rates of advance. A slow rate of advance in temperature is important to achieving equal temperatures throughout the glass. Even using 3mm glass, the rate of advance might need to be as slow as 50°C per hour. The corollary of this is that you will not need to use as high a temperature to achieve the effect. Heat work means that it is not an absolute temperature that will achieve the effect. The slower you put the heat into the glass the lower temperature required. The understanding of this relationship will require experimentation to establish the relationship to the rate of advance and the top temperature required. For example, a satin polish of a sandblasted surface can occur at 650°C, if held there for 90 minutes.
In this case, a 50°C rate of advance will probably not require more than 600°C – and probably less - to achieve the shiny surface normally achieved at 660°C with a 200°C rate of advance. At 50°C per hour, it will take 12 hours to reach 600°C, although a little more than 3.25 hours at an advance of 200°C to reach 660°C. The input of heat acts upon the glass throughout the process, making lower working temperatures possible. The reduction in temperature required is not directly related to the reduction in the rate of advance. You will have to observe during the experimental phase of this kind of process.
If it was desired to fire enamels that mature at 520°C to 550°C you could put the sheets in vertical racks to allow the heat to get to both sides equally as Jeff Zimmer does. But this will only work for very low temperatures and for quick firings, otherwise the glass will begin to bend.
There are limits to this strategy of getting upper and lower surfaces to the same temperature, both in terms of physics and practicality. There are temperatures below which no amount of slow heat input will have a practical effect, for example, due to the brittle nature of the glass. Even where it is possible, it can take too long to be practical. For example, I can bend float glass at 590°C in 20 minutes into a 1/3 cylinder. I could also bend it at 550°C (just 10°C above the annealing point), but it would take more than 10 hours – not practical.