Tuesday, 31 May 2011

Installation of Glazing Bars

There are a few tips that concern the installation of glazing bars into wood frames. An important element to understand is that the purpose of the bars is to protect the panel from horizontal wind pressures on the window, not to lift the panel or in any other way strengthen the panel vertically.

The holes on one side should be at least 5mm deeper than the other. For a really secure attachment one side should be at least 15mm deep and the other 7-10mm. This allows a significant amount of wood to seat the bar. The bar should be at least 10mm longer than the opening is wide.

The hole you drill should be 1mm larger than the bar diameter. This will make moving the bar easier. Additionally, the ends of the bars should be filed to remove any roughness. Also greasing the ends of the bar with tallow or candle wax will ease the movement of the bars.

If the bar is to be installed inside sash windows you can ease the installation by determining the height of the hole to be drilled by presenting the panel to the opening and marking the frame where the bar is to be attached to the panel. Drill the hole so the edge of it is flush with the rebate. This allows you to use a chisel to open the hole enough to allow the bar to be placed in the socket now prepared. In these cases the bar needs to be no longer than the opening.

The installation should be completed by forcing putty into any gaps left between the bar and the hole. This will stiffen and help to firm up the bar’s attachment to the frame.

Friday, 27 May 2011

Cementing Panels

I recently had the occasion to repair a panel made by a friend of the clients several decades ago. It was cemented by pushing commercial putty under the leaves of the leads. It illustrates very well why lead light cement should be brushable to completely fill the space between the glass and the came.

This photo shows how the putty filled the space above and below the glass but not between the glass and the heart of the came.

This photo shows the putty missing from the corners of the glass. There has been a little chipping of the putty in the dismantling process, but not much.

The question may be asked about what is so important about a bit of putty missing from the edges of the glass, it is sealed along the leaves of the came. Yes, this style of cementing will seal the panel from the weather for a time. But had this glass been in a window instead of hung inside, it is questionable whether it would have begun to leak only about 20 years after being made. Certainly as the putty begins to break down, the moisture will rapidly find its way into the inside.

The only way to be certain that the panel is completely weather proofed is to use brushable cement. The cement is pushed under the leaves of the lead with a stiff brush. You know the fill is complete by the cement oozing out of the other side.

It is possible to make up a brushable cement from commercial putty. You simply add some white spirit to the putty. I make a depression in a fistful of putty and add white spirit. Fold over the sides into the well and gradually, the white spirit is mixed into the putty. Continue adding white spirit until you have a very thick molasses that can be pushed around with a brush.
Of course, while you are doing this mixing, you can add a blackening agent - powdered or oil based black pigments are best.

Monday, 23 May 2011

Jewellery-scale Ovals

Rather than trying to perform the difficult task of cutting small ovals, you can use the heat of the kiln to do some of the work for you.

Cut a rectangle the length and width of the oval you want. Then groze the corners to the approximate curve of oval you want. Do not worry about the little inaccuracies of the curve. If it is the curve you want, the heat of a full fuse will even out the edges into oval you want. Clean the glass, assemble and fire to your normal full fuse temperature. The result will be a smooth edged oval of the shape you grozed from the glass. Of course anything less than a full flat fuse will produce a piece with some of the inaccuracies that you grozed into the glass.

If you do not go to a full fuse, or are using only 3mm thickness of glass, this will not work.

Monday, 16 May 2011

Effect of Glass Weight on Slumping

Just as the mould size and shape have effects on slumping temperatures and strategies, so does the weight.

When slumping you are making use of the combined effects of gravity and the increasing softness of the glass. The same thing happens when you have a thick piece of glass as when you have a large span in the mould. As the weight of the glass increases, the temperature at which it will begin to slump is decreased. There is an inverse relationship between the weight and the slump temperature just as there is between increased span and slump temperature.

A 3mm piece will take more time or more heat to fully slump into a mould than a 9mm piece will into the same mould. Observation will give you the information on what the temperature differentials are.

Monday, 9 May 2011

Bowed Glass for Cabinets

This is glass which is slightly convex and normally found in multiple-paned cabinet doors. Glass workers are sometimes asked by antiques dealers to do a replacement.

You can make a mould and do a slump.

However, you should consider doing a drop out or aperture drop. Normally these are thought of as circular, but they can be of any shape you want. The reason for making them as a drop out is that the surface of the bent glass will be completely unmarked.

I have made these several times for antique dealers. To do it, make a rectangle in fibre board about 10mm larger than the glazing size. Place a piece of glass about 40mm larger than the rectangular hole and fire. You need to watch. It will begin to slump at around 520C - or less if it is not float glass. You need to go slowly so the glass does not drop too much.

You will know from the existing pieces how deep a drop is required. Measure that and place a witness to determine when the slump has gone far enough. This can be a piece of kiln furniture with fibre paper over it. It can be a reference point on the far side of the kiln. In my case it normally is a stack of fibre board pieces with fibre paper on top to build it up to the correct height.

When the glass is just about to touch the witness, flash cool the kiln to just above the annealing point and close the kiln. If the temperature rises back into the forming temperature range, flash cool again. Twice should be sufficient to ensure that the glass does not move any further.

Monday, 2 May 2011

Cutting Flashed Glass

Some recommend cutting flashed glass on the clear or non-flashed side. This is based on the idea that the flash is only laminated to the main body of glass. My view is that flashed glass has proved to be very stable over many centuries, and so is firmly a part of the whole sheet.

What is more important is to observe that flashed glass often has a bow. If you place the glass on the bench, you may find that it rocks or sits up from the bench. If you cut the glass on the convex side, that is the side which is not resting on the bench except at the edges, you may find that you break the glass during the scoring, unless you are using the lightest of pressures. It is more certain to get a good break if you score the glass on the concave side - that is where the edges are slightly raised from the bench. So the important element in deciding which side to cut is to score the concave side whether that has the flashed colour or not.

This does not occur with all flashed glasses, and is more important on large sheets than small ones. On the small ones, the curvature is so small as to be immaterial.