Tack Fusing Considerations

1 – Initial Rate of Advance

Tack fuses look easier than full fusing, but they are really one of the most difficult types of kiln forming. Tack fusing requires much more care than full fusing.

On heat up, the pieces on top shade the heat from the base glass leading to uneven heating. So you need a slower heat up. You can get some assistance in determining this by looking at what the annealing cool rate for the piece is. A very conservative approach is needed when you have a number of pieces stacked over the base layer.  One way of thinking about this is to set your initial rate of advance at approximately twice the anneal cool rate. More information on this is given in this entry. 



2 – Annealing 

The tacked glass can be considered to be laminated rather than fully formed together. This means the glass sheets are still able, partially, to act  as separate entities. So excellent annealing is required.

Glass contracts when it's cooling, and so tends to pull into itself. In a flat, symmetrical fuse this isn't much of a problem. In tack fuses where the glass components are still distinct from their neighbours, they will try to shrink into themselves and away from each other. If there is not enough time for the glass to settle into balance, a lot of stress will be locked into the piece that either cause it to crack on cool down or to be remarkably fragile after firing. In addition, in tack fusing there are very uneven thicknesses meaning it is hard to maintain equal temperatures across the glass. The tack fused pieces shield the heat from the base, leading to localised hot spots on cool down.

On very difficult tack fuses it's not unusual to anneal for a thickness of four to six times greater than the actual maximum thickness of the glass. That extended cool helps ensure that the glass has time to shift and relax as it's becoming stiffer, and also helps keep the temperature more even throughout.

So in general, tack fused pieces should be annealed as though they are thicker pieces. Recommendations range from the rate for glass that is one thickness greater to at least twice the maximum thickness – including the tacked elements – of the whole item. Where there are right angles - squares, rectangles - or more acutely angled shapes, even more time in the annealing cool is required, possibly up to 5 times the total thickness of the piece.

It must be remembered especially in tack fusing, that annealing is much more than the annealing soak. The soak is to ensure all the glass is at the same temperature. The anneal cool over the next 110ºC is to ensure this piece of different thicknesses will all react together. That means tack fusing takes a lot longer than regular fussing.

3 – Effects of thicknesses, shapes, degree of tack

The more rectangular or pointed the pieces there are in the piece, the greater the care in annealing is required. How you decide on the schedule to use varies. Some go up two or even four times the total thickness of the piece to choose a firing schedule.

A simplistic estimation of the schedule required is to subtract the difference between the thickest and the thinnest part of the piece and add that number to the thickest part. If you have a 3mm section and a 12mm section, the difference is 9mm. So add 9 to 12 and get 17mm that needs to be annealed for. This thickness applies to the heat up section as well.

Another way to estimate the schedule required is to increase the length the annealing schedule for any and each of the following factors:

·         Tack fusing of a single additional layer on a six millimetre base

·         Rectangular pieces to be tack fused

·         Sharp, pointed pieces to be tack fused

·         Multiple layers to be tack fused

·         Degree of tack – the closer to lamination, the more time required

The annealing schedule to be considered is the one for at least the next step up in thickness for each of the factors. If you have all five factors the annealing schedule that should be used is one for at least 21mm thick pieces according to this way of thinking about the firing.

4 – Testing/Experimentation

The only way you will have certainty about which to schedule to choose is to make up a piece of the configuration you intend, but in clear. You can then check for the stresses. If you have chosen twice the thickness, and stress is showing, you need to try 3 times the thickness, etc. So your annealing soak needs to be longer, if stress shows. You can speed things by having your annealing soak at the lower end of the annealing range (for Bullseye this is 482C, rather than 516C).

You will need to do some experimentation on what works best for you. You also need to have a pair of polarisation filters to help you with determining whether you have any stress in your piece or not. If your piece is to be in opaque glasses, you need to do a mock up in clear.