Tacking Freeze and Fuse to Base Glass

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. 

Sintering

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.

Scheduling the Rate of Ramp

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.

Remaining Parts of the Schedule

Sintering Soak

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.

Anneal Cool

The annealing soak and cool should follow the rates given for the calculated thickness - in this case for 21mm or 28mm.

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.

Freeze and Fuse

"Freeze and fuse" is a term devised to describe a technique to obtain complex edge shapes and some bas relief.

The basic method is as follows, although there are a number of variations that can be successfully adopted.

Mix enough water with fine frit to make a damp slurry.

Then place about 3mm into your mould and tap on a hard surface. Tap quite vigorously to bring any air bubbles to the top and compact your powder.

Use a paper towel at this point and blot off any water that has risen to the surface.

Continue to layer, tap and blot until you're level with the top of the mould.

The more you tap and blot out any water (and every time you tap, more will rise to the top) the better your results will be.

When your paper towel won't absorb any more water, you're ready to put your mould into the freezer. One to three hours should be enough, but it must be frozen throughout.

Take the frozen glass from the mould. Letting it sit while you programme the kiln will allow it to come from the mould more easily. Place the glass form on the kiln shelf. Raise the temperature as fast as you like to 90C. Soak there for at least half an hour to remove any water in the piece. Then raise the temperature at about 100C/hour (depending on the thickness and size of the glass form) to a low fire polishing or sintering temperature (about 720C to 740C). Higher temperatures will flatten the form and change its shape. Soak at this sintering temperature for an hour or so.  Check on the progress of the firing by peeking at 10 minute intervals and advance to the next segment of the schedule when  the surface begins to shine.  

If you are planning another firing, you should not fire beyond the first hint of a shine appearing.

Some experimentation is required to get the best combination of rate, time and temperature.

Experience will show you variations on this basic method.

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

Freeze and Fuse

The object of this technique is to make a shaped piece without use of a refractory mould. It is applicable to small items.

You can use jelly, soap, candle, etc. moulds. They can be rigid or flexible. They should be without undercuts and have a draft, which is why jelly, soap, and candle moulds are so suitable. Be careful of the size, as a large amount of frit can be required even for a small mould.

Some people use only powder for this process. I use a 50/50 combination of powder and fine frit. You can use clear frit with powder. If you do so, you need to measure out the appropriate amounts.  Then put the frit and some water into a container with a closure. Close and shake to wet the frit. Then add the powder and shake again to ensure the powder adheres to the frit. Once thoroughly mixed, add more water to make a thick slurry.

Pack the mould with the mixture. Then using absorbent paper towels firmly pat the contents of the mould as dry as you can. Place the mould in the freezer for at least a couple of hours, or for large ones overnight.

When frozen, remove from the mould and place on the kiln shelf. The shelf needs a separator which can be kiln wash or fibre paper. Some leave the piece to thaw out and some more of the water to evaporate.

Whether you fire immediately or let the piece thaw and evaporate, you need to fire slowly to 100C and soak there until no more moisture is evident to avoid creating pockets of steam that will blow the piece apart. After that you can fire as normal for an initial firing of a two layer piece.

The piece will shrink a bit during the firing, but it is safe to anneal for the original thickness of the frozen piece. If you have a large piece or one with lots of variation in thickness, you should use at least the next thickness up from the Bullseye tables for annealing thick pieces. Sometimes you should use two steps up.

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