Re-firing

A frequently asked question is “how many times can I re-fire my piece?”

This is difficult to answer as it relates to the kind of glass and the firing conditions.

Kind of glass

Float glass is prone to devitrification. This often begins to appear on the second firing. Some times it may be possible to get a second firing without it showing. Sandblasting the surface after getting devitrification will enable another firing at least.

Art glass is so variable that each piece needs to be tested.

Fusing glasses are formulated for at least two firings, and experience shows may be fired many of times. The number will depend on the colours and whether they are opalescent. Transparent colours on the cool side of the spectrum seem to accept more firings than the hot colours. Both of these accept more firings than opalescent glasses do.

Firing conditions

Temperature

The higher the temperature pieces are fired at, the fewer re-firings are possible. So if multiple firings are planned, you should do each firing at the lowest possible temperature to get your result. This may mean that you have relatively long soaks for each firing. The final firing can be the one where the temperature is taken to the highest point.

Annealing

You do have to be careful about the annealing of pieces which have been fired multiple times. A number of people recommend longer annealing soaks. However, I find that the standard anneal soak for the thickness is enough. What is required is cooling rates directly related to the anneal soak.  This is a three-stage cooling as described in the Bullseye chart Annealing Thick Slabs.  The slump firing can be annealed at  the standard. 

Slumping

In general, slumping is at a low enough temperature to avoid any creation of additional stress through glass changes at its plastic temperatures.  But any time you heat the glass to a temperature above the annealing point, you must anneal again at least as slowly as in the previous firing. Any thing faster puts the piece at risk of inadequate annealing.  Of course, having put all this work and kiln time into the piece, the safest is to use the cooling rate as for a piece one layer thicker.  My research has shown that this gives the least evidence of stress.

Testing

Testing for stress after each firing will be necessary to determine if there is an increase in the stress within the piece. In the early stages of multiple firings, you can slow the annealing and if that shows reduced stress, it will determine your previous annealing schedule was inadequate. When reducing the rate of annealing does not reduce the stress, it is time to stop firing this piece at fusing temperatures.

Revised 6 May 2023

Measuring Openings

Measuring rectangular openings

Timber and metal openings can vary in their dimensions. So measure each opening at the top and bottom, and at the left and right as well as the middles of each side and top and bottom.

To be perfectly sure the window has right angles – is square – take the diagonals from opposite corners, e.g., top left to bottom right and top right to bottom left. These measurements should be within 5mm of each other for the window to be considered square.

You can check for ”squareness” with a try square, although that is not completely accurate.

Templates

If it is not possible to tell where the right angles of the opening are, a template is called for. The material to be used for taking templates should be stiff, easy to cut, unaffected by moisture, and relatively inexpensive. This eliminates paper and some cardboards. If you can find stiff corrugated cardboard this works well. Mounting board works well too, but is expensive. Foam board is excellent, but also expensive. Hard board or other thin pressed board is inexpensive but difficult to cut with a knife. Thin plywood is also a good material for templates, especially if the opening is relatively regular. The more complicated the opening, the more cardboard, mounting board, or foam board becomes useful for its ease of shaping to the opening.

What ever material you use, you must mark which is the interior and exterior and for further checks, which is left and right. Fit this template into the opening to make sure it fits into the opening smoothly. This template will form the external extent of the built window when it is installed into an opening with a rebate.

Where the window is to be fitted into a channel, as in stone, you need to make the template of stiff material so you can determine the panel can be installed and that there will be enough of the panel within the stone channels to ensure the stability of the window in the future and still be able to manipulate the leaded panel into the opening.

Direct or Trace cutting

Place the glass over the pattern and run the cutter along the pattern lines you see by looking through the glass. There's no need to draw lines on the glass. For translucent glass you may need a light box.

You should be aiming to cut glass efficiently and accurately. Trace cutting is the most efficient, as it completes in a single operation what other methods –such as drawing on the glass or making templates from the cartoon - take several steps to accomplish.

It is more accurate because each extra step required for other methods increases the possibility for error. The fewer times you copy the original pattern lines, the less likely you are to diverge from the original pattern lines.

It is very important to keep the cutter at right angles to the glass - as seen from side to side, not vertical.  This of course is true of all cutting.  It makes the cutting inaccurate, because the light is bent when coming through the glass much like water changes the apparent angle of sight into its depths.  Tilted cutters also have undesirable effects when breaking the glass.

Templates of openings, 6

When the opening is in stone, slight variations occur in the process of taking a template. The main difference is that the rebates are concealed. The rebates are slots into the stone. Thus, the template must slip into the slotted rebate. In these cases, the stiffer the material being used to take template, the better. Usually, thin plywood is the best material, as it has to be manipulated many times and in ways similar to the final panel.

Things are further complicated, as tracery is more common in stone than in timber framed openings. A complex opening shape may require two or more parts to enable the panel to be inserted. The taking of a template will help greatly in figuring out how the panel will be inserted into the opening.

Additionally, when the template is in position, you should mark the visible portion of the opening onto the template. Mark which is the inside and which the outside. Finally, mark on each template which side has the deeper slot as this will help in installation.

Templates of Openings, 5 – Irregular Openings

Irregular openings such as trefoils and other tracery need to have templates taken with consideration on how the final panel can be put into the opening.

In the cases where the whole of the rebate is exposed, it is normally possible to put the panel in as a single whole piece.

So, the template is taken as for any other opening. It is more complex and time consuming as there are so many more sides than in a simple rectangular or circular opening.

Templates of openings, 4

Round headed openings can be considered as a special case of a circle.

The horizontal you must find is the shoulder of the window. This is the place from which the curve springs on each side. The opening is generally vertical up to this point and then begins the curve.

You need to make sure you have marked where this shoulder is on the template. You should indicate any reference points from the frame onto the template.

The join to the lower part of the window must be made obvious. Normally there will be an overlap between the lower rectangular template and this approximate half circle. You need to mark where this overlap occurs, if you do not fasten the two sheets together. This can be done by marking across the two sheets in a few places. This will enable you to join them exactly back at the studio.

Templates of openings, 3 - Circles

Occasionally the window is circular and sometimes an oval. In both cases a template is important. The circle rarely is exact. Take the template in the normal way and then ensure you mark the verticals and horizontals for the opening. You often can use the jointing in the woodwork to help with these. Also mark any other reference points from the opening. Finally, mark which is the outside and which the inside.

This procedure will ensure that you will be able to fit the panel into the opening.

Templates of Openings, 2

Irregular rectangles

If you have found or can see that the opening is not a true rectangle and cannot determine where any right angles are, you need to take a template.

The objective is to make a piece that will fit into the opening without bending or being too small for the space. It will be the same size as the finished panel and so you will be able to put the finished panel into the opening without needing to trim or expand the panel.

First, trim the sheet of material you have chosen to use to a size a little larger than the measured size. Place the uncut side along one of the long sides of the opening. If the opening is a portrait format, place it on the right or left side as convenient to you.

Next, adjust the bottom by marking a line on the sheet. This is where a second person is very useful. One person can hold the sheet in place on outside of the opening and the other do the marking from the inside –in the case of the rebate being on the outside and vice versa if the rebate is on the inside. The marked line should be as close to the edge of the rebate as possible. The special case of an opening in stone will be dealt with separately.

Then take the sheet to a place where it can be safely cut. A long metal straight edge and craft or “Stanley” knife are often the best aids to cutting straight lines. Replace the sheet into the opening after cutting, and make any adjustments to the size and angles of the sheet at the bottom by marking and cutting as necessary.

When the side and bottom are adjusted, start on the other side. Proceed as for the bottom.

When the side is finished, start on the top.

Finally, present the whole sheet to the opening to make sure it slips into place with no snags, or bending of the sheet.


It may be that the opening is too large for a single sheet. In that case you will need to work with two or more sheets and try them together for the final fitting into the opening. You can put them together in the window. You can fasten them together with tape or other fasteners to make one sheet. You can also make two parallel lines both at angles and at intervals across the sheet so that when you get back to the studio you can exactly reproduce the full sheet by matching the marks and then firmly fastening them together. This makes transport of large templates much easier.

You will know that a panel made to a template made in this way will fit into the opening, no matter how irregular the opening may be.

Templates of Openings, 1

If you have an irregular opening, it may be best to take a tracing of the opening. Usually this will be in a larger opening and so a helper may be necessary to hold things.

The material used to take the template must have a few characteristics:
- It must be stiff enough to have the minimum possible bend over the width or height of the opening
- It must be easy to mark with a pencil or other implement
- It must be easy to cut or shape
- It should be light to make it easy to lift it to the opening for the many adjustments that will be required.


A number of materials can be used: stiff card, mounting board, corrugated cardboard, thin plywood, and many other sheet materials. I have found stiff corrugated cardboard easy to use.


More information in this series is at:
Irregular rectangles
Circles
Round headed windows
Irregular shapes
Stone

Cutting with Patterns or Templates

As a general rule, always make the hardest cut first. Glass tends to run in a straight line. This means inside curves should be done before any other cut is made.

It will be most efficient to place the pattern to be cut with the inside curve facing the raw edge of the glass. If something goes wrong, the pattern can be backed away and tried again, resulting in less glass and time wasted.

It is also hard to break very thin strips of glass away from the desired piece without getting ragged, chipped edges. Allow a 6 mm minimum distance from the edge of the glass when placing the pattern on the glass unless the edge is going to be used for the whole edge of the piece.

When scoring around a paper pattern it is necessary to steer (turn) the cutter in the proper direction. The paper will not turn the cutter for you. You should steer the cutter by turning your upper body rather than your fingers, wrist or elbow.  Failing to do this may allow the cutter to run over the pattern and so fail to score the glass.

Alternatives to using the paper pattern directly as a guide in cutting glass are to outline the pattern paper on the glass with a pen or to draw the pattern on the glass while it overlays the drawing. You follow the inside edge of the pen line with the cutter.

Directly cutting the glass over the cartoon avoids the time spent in making patterns, and the difficulties and inaccuracies in multiple transfers of the shape. Often a light source is required under the cartoon to enable the lines to be seen through the glass. There will always be times when the glass is so dark or opalescent that the lines cannot be seen and therefore a pattern is required.

Keep the pattern cutting restricted to the times when nothing else will do.

The only times I use patterns for cutting are when the glass is too dense for the cartoon lines to be seen through the glass with light behind or for repeat shapes where a pattern can speed the process.

Two pattern pieces used to draw repeats of sails on the glass and then cut out

A boat design assembled in the kiln

 I draw around the pattern pieces, as that avoids the possibility of the cutter riding up on the card that I use for templates.  This comes from several occasions when the cutter did go over the template which prevented the score and so created a bad break.  Others do score around the template stuck to the glass successfully.