What Formations are Made in Drapes?

The shape glass takes up during a drape seems to vary depending on whether the glass is a square or a circle at the start. The square often forms a taco shape before the “horizontal” ends soften and fall, pushing the glass into four drapes. The angle of these drapes will move more toward being straight down with additional time or heat. Circular draped glass tends to form three drapes, which also become more vertical with time and heat.

Early stages of a steep drape

One occasional problem with elevated moulds is that the draping glass is “longer” than the mould is tall. Understanding how the glass forms during a drape will help avoid this effect. The glass drape first forms a curve as it begins to drop, which continues to be in place until it touches the mould. The rest of the firing stretches the bow in the glass out.

If there is no mould at the bottom for it to touch because the mould is elevated, the curve goes beneath the bottom edge of the mould.

If the draping glass is longer than the elevated mould, the glass will drape to the shelf and can creep under the bottom edge, trapping the mould. 

 Alternatively if the mould is not elevated, the glass will flatten on the shelf. The flattened ends will spread outwards, but sometimes also creeping under the edge of the mould, trapping the mould.

The glass can be the correct length for the elevated mould, but the time and temperature combination scheduled allows the whole drape to stretch at the base, so the tips of the drape create one or other of the trapping effects.

The initial prevention of trapping the mould is to measure the length of the drape. The size of the blank should be no longer than double the length of the sides plus the width of the supporting top (the base of the mould when stood open end up). This length is the maximum diameter of a circle. 

 The diagonal of a square blank needs to be no longer than that either.  A simple visual method is to establish a square corner directly onto the glass.  Draw a 45° angle line and measure off the length on the diagonal. Draw a right angle line from the base to the point on the diagonal and you have the length of the sides ready to score.

It should be remembered that the glass will stretch to some degree during the drape. The length of that stretch is dependent on the temperature and time, so observation by frequent peeking will give the results wanted by either skipping to the next segment or extending the hold.

There is more information on scheduling for drapes in the e-book Low Temperature Kilnforming available from Bullseye  and Etsy.

Peeking

Observation is desirable for learning and essential for several processes.  How you do that is important to the safety and health of your eyes and skin.  This post gives some guidance on the protections required.

Each observation should take a fraction of a second. It is called peeking to distinguish it from looking or watching. There is a method to doing this. Think about what you are looking for before you open the kiln. Pop open the kiln to record with your eyes, close the kiln. Think about what you saw. If necessary, repeat.  But only after you have thought about what you saw.

Do not spend time looking into the kiln.

Think about the necessity for observation before buying your first or next kiln. 

The best kilns are those with generous observation ports, both in number and size.  These allow you to peek into the kiln without creating a draft within the kiln.  Two or more ports are best, as you can shine a light into one of them to illuminate the interior of the kiln at lower temperatures.

If you do not have ports you will need to open the kiln. This is easiest to do with top hat kind of kiln.  The top hat kiln keeps a lot of heat in the upper portion of the lid, making the amount of heat dumped less than on other kinds of opening.  You can peek in at the level of the shelf, so minimising the amount of heat being dumped.

The problems with lid opening kilns, requiring you to peer down into the body of the kiln, or with doors opening to the front, is that you are dumping a lot of heat directly at yourself.  You also are losing a significant amount of heat from the kiln.  The large air exchange also will disturb any dust in the kiln and that may fall onto your work.

It is possible to make ports in your kiln by drilling a large diameter hole in the side of the kiln and through the insulating material (assuming you do not have side elements).  This post gives some ideas.

In all the cases where it is necessary to open a lid or door, you must close the kiln slowly and gently to minimise disturbance of the air within the kiln.

The effects on the glass of peeking at various temperature ranges varies between the rise and the fall in temperature.  This post gives some ideas of the effects.