Ramp and Anneal Rates for Tack Fusing

Tack fusing is more difficult than most realise.  Many failures – usually breakages – occur because the complexity of tack fusing is not fully acknowledged.

Ramp Rate 

Calculations

One of the effects is the slower rate of advance that needs to be used.  The rate of advance needs to be slowed to that applicable to 1.5 to 2.5 times the actual total thickness of the assembled piece. 

Reasons

The reason for this firing for apparently excess thickness is the shading effect of the overlying pieces upon the glass below.  Glass is affected by radiated heat, whether the heat comes from above or the sides.  The parts of the base glass that have glass on top cannot receive the radiated heat.  This means the shaded base glass needs time for the heat to be conducted through the overlying glass to it. 

Beginning of heat input

Progress of heat input showing some parts of the base are compeletly heated while others are not

Glass is a good insulator, resisting any heat transmission through overlying glass. Slowing the rate of advance allows the convection of heat to the lower levels to be adequate to avoid heat stress.  The reason for the 1.5 to 2 factors is that experience has shown a simple applied arrangement will be safe with a factor of 1.5 as the calculated thickness.  If you have stacks or lots of difference in thicknesses, you need a slower rate to allow for the conduction of heat.  This is where the 2 times actual thickness factor is useful.

Finding the Ramp Rate

The information on the rate of advance for evenly thick pieces of 6mm to 9mm is widely available.  Determining the rate of advance for thicker items is more obscure.  You can get some guidance from the manufacturers’ websites.  But where the guidance is for thinner pieces or it is unclear, you need to find another reliable source. 

One very reliable source is the Bullseye annealing chart for thick slabs.    Yes, this chart tells you about the annealing of thick items, not about the ramp rate to the working temperature.  But you can infer the initial rate of advance from the final cooling rate.  The principle is that the glass can survive the indicated cooling, so it should also survive that rate of advance from cold to working temperature.

This means that a set up of a 6mm base with two layers of glass pieces on top distributed around the base, is a total of 12mm.  This should be fired as though 18mm (1.5 times actual) or up to 30mm (2.5 times actual).  In the first case the chart indicates the final cool rate is 150°C per hour.  This can be used as the initial rate of advance to at least 540°C (above the annealing range).  If you choose to use the 2.5 times factor, the initial rate will be 65°C per hour.

This approach gives you a reasonable degree of certainty about how fast you can fire your glass from cold.  Note that you still need to have a conservative bubble squeeze segment in your schedule, especially if the lay up includes areas where air might be trapped.

Annealing rates

Annealing times and rates are normally dependent on the thickness of the fired glass.  But published annealing rates are based on both even thickness across the piece and on cooling from two sides – i.e., not on the floor of the kiln.

Calculating for even thickness

If you have taken your stacked piece to a full fuse, you can anneal for the final thickness.  I would be a little more cautious with a contour fuse and anneal as though it were three to six millimetres thicker than when completely flat because you cannot be certain that the piece is evenly flat unless you obeserve.

Calculating for tack fused

If, however, you are firing to a tack fuse you need to look to schedules for thicker pieces.

Reasons

Glass remains an insulator as it cools.  As glass cools, it must conduct the heat through the thick parts at the same rate as through the thinner parts to avoid inducing stress.  Remember the principle of annealing is to keep all the glass with 5°C or less difference in temperature.  The thinner glass gives its heat up quicker than the thick.  This will induce stress and it can be enough to break the glass in the kiln or, more usually, some long time after the glass is cool.  This means you need to control the cooling to a rate that would be suitable for thicker glass. 

At the beginning of the cool the heat loss is from the surface and to a lesser extent through the shelf.

Further heat loss shows the exposed base layer is giving up its heat throughout, although other areas are only beginning to cool.  It will take some time for the three layer stack to cool.  The uneven cooling leads to the introduction of stress.

Determining the rate

The annealing soak length and the rate of the annealing cool are directly related to the thickness calculated for your piece.  You have already chosen a calculated thickness for the rate of advance to avoid breaking the glass.  Use the rates given in the chart for that thickness for your soak and anneal cool.  Any annealing with a shorter soak and a faster cool risks inducing stress and possible breakage.

Rates of advance and annealing are intimately connected.  A tack fused piece must be annealed as though it were 1.5 and up to 2.5 times the actual total thickness.  Annealing of tack fused pieces cannot be skimped.

Further information is available in the ebook Low Temperature Kiln Forming.