Annealing Thickness

Factors relating to how to determine an annealing soak time and cool rate are numerous. The thicker the piece, the more conservative you need to be in the soaking and cooling. This is because the annealing soak time increases almost exponentially with the thickness of the piece. The reduction in the speed of the anneal cooling has an approximate exponential relationship also – requiring much slower rates of cooling.

In the simple case you use the smallest dimension to determine the thickness for annealing. The heat is assumed to travel the shortest distance to dissipate. Thus a piece cooled from both sides will be taken as the simple case. This needs to be modified in a number of circumstances.

1 - Pieces cooled from one side, as where the shelf is heat retentive, or fired on the base of the kiln can effectively cool from only one side. This doubles the distance the heat has to travel. So you need to use the numbers from any annealing table for glass twice the thickness of the actual piece.

2 - Then if you have variations in the thickness of the piece, you need to be more careful, because the thinner areas will cool faster than the thicker, giving the potential for stress creation.

3 - If you are doing anything less than a full fuse, you will need to be more careful. If the pieces are tack fused, they will in part, act independently of each other and so need more careful, slower annealing.

4 - The less symmetrical a piece is, the greater care needs to be taken, just as with differences in thickness.

5 - The heavier or more insulating the refractory materials that surround the piece, the greater the care needed in annealing.

A rule of thumb is to consider these variables and add to the soak times and reduce the speeds of anneal cooling in accordance with the number of variables exhibited in your piece. With the first of these variables - cooling from one side - use the schedule for twice the actual thickness of the piece. Then add another step up in thickness for each additional variable.

So, as an example, the most common complicating factor is the inability of the kiln to cool the pieces equally from both sides. This requires you to double the thickness to read an appropriate schedule from the table making the schedule for 12mm glass.

But your piece of glass also varies in thickness, so you need to add another step up in thickness, giving you 19mm.

However you are not intending to go to a full fuse, so you need to add another thickness. Now you are up to the table for 25mm.

But the pieces are asymmetrical, so another step up in thickness is required giving you 38mm.

And you have heavy dams under and around the piece, so yet another thickness level is required to read an annealing schedule from the table. This gets you up to 50mm as the part of the table you should be reading from. So you will need an 8 hour soak rather than 1 hour and need to reduce the temperature at 4C/hour rather than 80C/hour.

Of course, not every project has all these variables in it, so you use only those that apply, but each of those that do apply requires a step up in thickness to read the schedules for the numbers to plug into your controller.

This is a rule of thumb, so you will need to test any pieces for stress and learn from your experience if this is conservative enough for you pieces.

Copper Backings

“Is it possible to to fuse copper to the back of glass?

The easy answer is - no.

But it can be done. There are a number of conditions that will help.

The copper needs to be thin and flat. It works best if you clean the copper of any oxidisation, 
and then coat it with borax or other devitrification spray that can act as a glass flux.

The fusing has to be done with a long soak to ensure the bottom of the glass is as soft as the top to assist the attachment of the copper. The devitrification solution will help soften the glass next to the copper sheet. You also have to protect your shelf from contamination by the copper sheet. This can be done by using 3mm fibre paper under the copper.

Not all attempts will be successful, showing that this process is on the edge of acceptability.

It is easier simply to glue the copper to the back.

“Does it matter whether it has been fused already?”

The glass does not have to be fused prior to attempting to attach the copper to the back. If it has been fused, you need to run a slower schedule than when fusing glass for the first time. A schedule for slumping, but with a higher target temperature – at least fire polishing – will be required.

Ramp Speeds for Slumps and Drapes

I find that a steady slow heat up gives best results for both slumping and draping. Many do a two stage heat up with a slower initial rise to a temparature above the annealing point and then faster afterwards.

A slow steady increase in temperature allows all the glass to be nearly the same temperature both across the piece and from top to bottom.

Speed in draping, especially where there are different colours and thickness, promotes "wings" as any unevenness in the heat across the piece will translate into variable drape speeds. Allow all the glass to get to the same heat, by going slowly.

I proceed at the same speed from start to the forming temperature - no speed up at all. Yes, the glass will accept it a rapid heating to the target temperature, but it does not promote even slumps or drapes.

Candle Bridge Moulds

A candle bridge mould is one of the most difficult moulds to use successfully. The problems relate to the kinds of work you are trying to do, the size of the glass to put onto the mould, the shape of the piece and the stability of the resulting piece.

With a candle bridge you are trying to do a combination of slumping and draping at the same time. You are slumping into the middle and draping over the curved sides.

Additionally the candle mould requires the glass to fall into a small opening and this requires long soak times. Long soak times mean the glass that is draping stretches while the central portion is trying to fall into the opening. Of course, if you don't want the depression to be flat, you don't have to soak so long and the stretching effects on the draping part of the of the glass won't be so great.

It would seem logical to measure the mould around the drapery curve (or arc of the mould) and to the shape of the ends, but experience has shown me that this leads to glass that is too long along the sides and bent at the ends. So I cut my blanks for candle moulds as a rectangle without curved ends and then round the corners of the rectangle just a little by nipping them with my grozing pliers before fusing.

If you measure along the top and along the length of the mould you have a piece of glass that will be increasing in length at the draping part of the mould, meaning that it will fall off the curve and onto the draft (or side) of the mould. The draft is an angle from the vertical. Good moulds are made with a draft so that if glass were to fall over the edge it still will be possible to get the glass off the mould.

The draft on a mould means the diameter of a circular one is greater at the base than it is at the rim. And it is common to measure only the diameter at the rim.  In the same way the dimensions at the outside base of a rectangular mould are larger than the rim of the mould.

Back to the rectangular candle mould. The draft on this means that measuring the base of the mould is slightly wider than the curved part of the mould, but less than if measured around its curved portion.

Experience has shown that in the case of the candle moulds measuring the width of the mould is sufficient. There is enough height in the moulds I have used that it does not make any functional difference if the glass does not reach the bottom of the curve on the mould. It is better than hanging off the edge.

The length of the glass should be no longer than the shortest part of the mould's length. Cutting a curve into the glass to allow a small overhang produces a depressed lip because of the length of the soak required for the slump into the small aperture of the candle depressions.

My soak for candle moulds is 90 minutes at my process temperature. This gives me a flat depressed area for the candle to sit, but it also means that the draping glass has been stretching. And it also means that the glass will drape unevenly as the various colours absorb heat differently allowing some parts of the glass to stretch more than others.

The placing of the glass on the mould is absolutely critical. It must be exactly parallel to the sides of the mould. Any slight movement from that will induce a twist in the resulting piece allowing it to rock. Arranging it exactly right and placing some kiln washed furniture at each side to keep it in place until it begins to slump is an important aid.  
The glass will begin to bend before it sticks to the kiln furniture.

I have never been able to get a stable candle mould whether from 3mm or 6mm thick glass. I always have to grind the base a little to make a stable piece. I take it as part of the process, but careful placing reduces the work.

Cutting Box Hinges

Cutting the brass tubing for box hinges is best done with a saw.

The manual version is to use a fine toothed saw - similar to those used by jewelers – with very light pressure.

The power solution is to use a Dremel or similar hobby motor with a cutting wheel attached. This cuts through the tube cleanly and quickly.

In both cases, a fine rat's tail file can clean up any burrs within the tubing.

If you use the internal tubing you can cut both at the same time. However, it is quick and easy to use the appropriate sized copper wire to insert and then bend down onto the corner of the box. This gives greater flexibility and avoids waste.

Vase Cap Fitting

“Can anyone offer me any tips for fitting the brass caps onto lampshades?”

There are at least three ways to get the right vase cap size.

Make up your shade in a cardboard mock-up. Use 3mm thick card or foam board to represent the glass, as the thickness of the glass is important in determining which vase cap is the correct size. Try your vase cap against the cardboard model, then if you need, alter the pattern so the glass pieces meet at just the right place to make the lip of the vase cap fit just over the top of the glass. You can do this by either shortening or lengthening the pattern a little at the top edge.

The second also involves making a cardboard mock up. After making this maquette, choose a vase cap that overlaps the top opening, covering all the edges. 

The third option is to use two vase caps, one above and one below the opening to clamp them together trapping the edges of glass between them. Use a furling and lock nuts with no solder at all to hold the lampshade together.

A general discussion of panel lamp dimensions