The question has been asked:
I'm wanting to add some freeze fuse pieces on to float
and just fire to a tack fuse … in one firing instead of two …[to avoid] losing
the detail on the freeze fuse pieces. The top temperature on freeze and fuse is
720°C versus a … float tack temperature of 787°C. [can this be done?]
My response:
What you are doing with the freeze and fuse process is sintering the
glass particles together by holding at a low temperature for a very long
time. This binds the glass together
without altering the overall shape of the object.
There is no reason why you cannot sinter the freeze and fuse
piece on top of a base glass, if you pay attention to one major thing. The freeze and fuse object will shade the
heat from the base glass. If you do not
slow the rate of advance enough, you will break the base glass by creating too
great a temperature differential between the part under the freeze and fuse piece and
the uncovered part.
Another element to be considered, is that the frozen object is damp. This will need to be dried by a slow ramp or it will further complicate the uneven heating problem.
Choosing the rate of increase in temperature is determined by the dimensions
of what is being sintered. One widely
practiced method is to double the total height and fire for that
dimension. For example, if the freeze
and fuse is 8mm high, add that to the 6mm base and fire for 28mm – (6+8=14)*2
=28mm.
Another slightly less cautious approach is to multiply the total
height by 1.5 and use the firing conditions for that thickness.
Determining the rate of advance for the thickness you have calculated
– by either method - can be aided by using the Bullseye chart for annealing thick glass. Look at the final cooling rate in the chart for the nearest
thickness. In this case, use the one for 25mm.
The cooling rate is given as 90°C per hour. If the glass can safely cool at that rate, it
should also survive that speed of heating at the start.
If you chose the 1.5 factor, the thickness to schedule for
will be 21mm. This is between the 19mm and 25mm thicknesses given in the Bullseye chart. The cooling
rate given for 19mm is 150°C and and for 25 is 90C. As 21mm is almost the mid point between the two, you can halve the difference in rates (150 and 90) to give 120°C as the rate of advance. Although in both schedules using these rates of advance for the described circumstance, I would add a soak at 250°C for 20 minutes, to be
cautious.
The length of soak for the sintering stage can be the same
as the soak for the freeze and fuse, as you will be both sintering the glass
pieces together and to the base glass too.
The Bullseye chart Annealing Thick Slabs can be used for all types of soda glass (which includes float glass) to determine the soak times and cooling rates. You only need to make alterations for the annealing temperatures. The annealing temperature I use for float glass is 540°C.
The first two stages of cooling are 55C each, so simple subtraction from the annealing soak will give the temperatures for each stage of the cooling. If we use the calculated 21mm thickness, the soak time will be 3.5 hours at 540°C. Then the Bullseye chart's displayed cooling rate of 20°C will apply from 540°C to 485°C, and the cooling rate of 36°C will apply from 485°C to 430°C. The final cooling rate of 120°C will be from 430°C to room temperature. The chart for these adaptations is described in the post about adapting the Bullseye chart for annealing. The reasons behind these operations are given in the ebook Low Temperature Kilnforming.
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