Monday, 25 June 2012

Complex Drapes

A question has been asked that relates to draping over a face mask mould, but this response relates to any drape with compound or multiple shapes.
The rate of advance should be a steady one all the way to the forming temperature. This should be 150C/hr or less. Thicker glass requires a slower rate of advance to allow the glass to heat all the way through. The point is to get the glass all the same temperature by the time it reaches the forming temperature.
Draping over an undulating mould takes more heat or time or both than simple drapes, because there is a much greater variety of form for the glass to conform to.
You have a choice about the top temperature and soak times. You can choose a low temperature with a long soak time, which most often leads to a minimum of marks on the underside of the piece, but requires long vigils at the kiln to determine when the drape is finished. The other strategy is to go for a higher temperature and shorter soak, which leads to more marks, but less time observing the firing. The higher temperature may be as much as 720C.
Note that there are a group of considerations about the size of the drape and the thickness of the glass being draped. 
There are some things you can do to assist the progress of the drape. One, already mentioned, is to increase the forming temperature.
Use a longer time, or as much time as required. Watch the draping progress because it is never certain how long the piece will take to conform to the mould sufficiently.
Use props. Place the props where the glass first starts to form just barely supporting the edge of the glass. As the glass begins to bend, it will slip off the supports - assuming they are well covered in separators, even pieces of kiln paper. This means the folds will start somewhere else than at the nose on a face mould or other high point on any other form.
Manipulate the glass. Reach in with wet wood sticks and push the glass about. One stick will be needed to keep the glass in place while the other pushes the glass about. The sticks do need to be both wet and strong. If you use dry sticks they will mark the glass as well as go on fire. Of course, you need protective gear to avoid burns to your skin and hair if you do this.