Single Layer Slumping

Almost all glass can be slumped as a single layer, whether produced for kiln working or not.  A few are extra sensitive at even slumping temperature and change character at around 630C-650°C, but all others can be slumped.  This posts concentrates on slumping of single layers of non-fusing compatible glass, but most of these elements can be applied to fusing compatible glass too.

The things you need to take care about are:

• Temperature
• Soak Times
• Edges
• Devitrification
• Annealing
• Testing

It certainly is possible to slump single layers. The resulting glass will be slightly less robust than two or more layers of glass, but simply because it is thinner.

Temperature

The temperature that you use needs to be high enough to allow the glass to take the shape of the mould, but low enough that the glass does not distort or stretch and thin.  This is a balance that you can achieve through observation of the firing. 

It most often is best to use the lowest practical forming temperature that you can.  Practicality here is about how long you want to wait for the glass to conform to the mould.  It is possible to take the glass to about 580°C and soak for multiple hours, but not very practical.  It does depend on the glass as to the temperature to be used for the slump.  There are two sources here that can help: the slump point test  and this table of glass characteristics. 

Soak times

A practical soak time will be 30 – 90 minutes, which will avoid marking the underside of the glass.  This means that the temperature will need to be lower than the softening (or slump) point of the glass. Your slump point test will tell you the temperature at which the glass begins to deform.  That is the best temperature to use.  If it is taking too long, advance the temperature by about 10°C.  If you used the table of glass characteristics to find a softening point, reduce that temperature by about 30°C as a starting point.

Edges

The temperature that you will choose to use is not high enough to allow the edges to change as they would in a fuse.   This means that you need to have the edges exactly as you want them in the finished project.  This will require cold working by hand or machine.  Neither will take a long time, but require the correct tools. This post gives you the comparison of fused and cold working methods.

Devitrification

While most glass can be slumped you need to be careful with opalescent glass, as it can devitrify easily.  Most wispy glasses are fine, but the more opalescent wisps they have, the more difficult there may be.  Streaky and single colour glasses are usually fine. 

Annealing

Another element in slumping glass not formulated for kiln working is the annealing of the glass after the slumping.  The annealing temperature can be estimated as 40C below a low temperature slump of a 280mm span of glass. The slump point test mentioned earlier will help determine the annealing point. You need to soak for a time - maybe 30 minutes - at the estimated annealing temperature and then cool slowly in case you have miscalculated on the annealing temperature.  In any case, a long slow anneal cool will pay dividends in a more robust glass.

Testing

You will find some manufacturers’ glasses are less adaptable to kiln forming than others.  So, it is best to run tests on the glass before committing to larger projects.

Remember TADSET - temperature, annealing, devitrification, soak, edges, test.

Edge Treatment in Cold Working

Frequently people who are grinding the edges of bowls, aperture drops and other vessels that need to have a smooth rim find that they are getting small chips of glass coming from the edge of the ground part of the glass.

There is a way to prevent theses unwanted chips  

The long established practice of glass workers has been to give the glass an arris at the end of each grinding stage before they change to a finer grit.  This small area of angled glass, allows the continued smoothing of the glass without creating such a sharp edge that the glass there is not strong enough to resist the grinding action.  

You will notice on a bowl or other rounded vessel, that the chips are almost always on the outside. The inside of the rim normally has an oblique angle to the rim, and the outside an acute angle.  The explanation is held in the angle.  As the rim is ground down, the outer acute angle becomes very thin as well as sharp.  At some point the glass is thinner than the grit used to grind the surface.  This causes little chips of glass to break off the edge.

By creating an oblique angle at the edge of the grinding surface, the glass will remain thicker than the grit being used to grind the glass.  If you feel you are taking off a lot of glass, it is advisable to check that the arris is still in place.  If not, give it a light grind to maintain the arris while using that grit. 

At the end of each stage of grinding, you need to add an arris for the next stage.  The reason for doing it with the coarser grit rather than the one you are about to proceed to, is that it maintains all the grinding at the same stage, enabling the whole piece to be finished to the same level of polish.

Wipe the surface dry and add marks with a paint marker.  Allow this to dry while you change grits.  The purpose of the marker is to assist you in determining when you have ground out all the previous marks, by the elimination of the paint.

Assembling Foiled Pieces

Keeping foiled pieces together while assembling them prior to soldering is sometimes a problem.

If the panel is rectangular - or at least one with multiple straight sides - you can use short battens of the length of the sides. Nail or screw them down to a board so that about one half of the panel is contained. So if it is a rectangle, two sides will be enough. If it is six-sided ,three pieces would do.

An assembled piece illustrating the two battens - although with a leaded panel

For ease of assembly, a copy of the cartoon should be fastened to the board first and then the battens fixed on top of the cartoon. The pieces can then be placed against battens and held there with pins or nails until the next pieces are ready.

Illustration of the cartoon fixed by the surrounding battens

Some prefer to tack solder the pieces together as they foil. This can be done in combination with the use of battens. However, leaving the soldering iron idling while foiling and using it only occasionally is very hard on the iron's tip and your electricity bill. I prefer to assemble the whole and then solder all at once.

Illustration of placing pins all the way around an oval panel

For circular or irregular shapes a slightly different approach is required. You can use multiple pins or nails along the perimeter to hold the foiled pieces together. An alternative is to cut a piece of scrap window glass to the shape of the external perimeter of the panel. Hold it is place with nails or pins and proceed as with a rectangular shape.

Placing nails all the way around an irregularly shaped piece

Refining Edges

You can make the freshly cut glass safer to handle by gently wiping the edges of the cut piece with the waste piece. This removes the sharpest edges without chipping the glass.

After the glass is scored and broken, you can remove small, unwanted chips with grozing pliers. The serrated jaws of these pliers are used to gently nibble away at the jagged edges.

Rough edges can also be smoothed with a carborundum stone. You rub the stone along each edge, upper and lower, to remove any sharp edges. You can remove more glass with the stone if you wish by a little more aggressive grinding action or just a longer light rubbing of the stone against the edges.

A diamond smoothing pad removes glass in much the same way as a carborundum stone, but does it more quickly with the coarser grades. You can use a number of grades to get an almost bright polish to the edges. These pads must be used with water.

A glass grinder is used by many people. Many models of grinders are available. The grinding surface of the bit is covered with fine diamonds, which grind away unwanted glass very quickly without chipping the edges. In addition, they are water-fed which keeps the glass from cracking due to heat, prolonging the life of the diamond bit, and preventing the powdery ground glass form flying around.

A glass grinder is not a substitute for accurate cutting.