Slumping a Form Flat

There are a variety of reasons that you might want to make a formed piece flat again for another kind of slump or drape.

There are lots things you think about when preparing to make a shaped piece flat.

I am going to assume there are no large bubbles in the piece.  You can see the posts  Large bubbles and Bubble at bottom  on the causes.

The following comments are things in five groups to consider when contemplating flattening an already formed shape.

Shape/form

• ·         Shallow forms with no angles have the fewest difficulties.  Take it out of the mould, put it on the prepared shelf and fire to the slump temperature.  Observe when it is flat and proceed to the annealing.
• ·         Forms with angles or multiple curves are a little more difficult.  If the piece has stretched in some areas to conform to the mould, you will have some distortion in the pattern and possibly some thinner areas.  It should be easy to flatten pieces on a prepared shelf with the same schedule, but a slightly higher top temperature as used to slump it.
• ·         Forms where the sides have pulled in will become flat, but continue to have curved sides.
• ·         Deep forms are possibly the most difficult.  The glass may have stretched, giving thin areas.  It may be that the process of flattening the glass will cause a rippled effect as the perimeter of the piece is a smaller size than the original footprint.  These deep forms are the least likely to flatten successfully.

Orientation

• ·         Which way up? Upside down or right side up?  Shallow forms are easiest to flatten by placing them right side up on a prepared shelf.  For deep or highly formed pieces, it may be best to put it upside down to allow the now higher parts to push the perimeter out if it is necessary.

Thickness

• ·         Thick glass will flatten more quickly than thin glass, so you need to keep a watch on the progress of the work to avoid excess marking of the surface of the glass.
• ·         Very thin pieces are likely to develop wrinkles as they flatten.  Even if they do not, there will be thick and thin areas which might cause difficulty in subsequent slumping.
• ·         Tack fused pieces are likely to tend to flatten at different places and times due to the differences in thickness and therefore weight. This makes shallow forms easier to flatten.

Temperatures

• ·         In all these processes, you should use the lowest practical temperature to flatten.  This means that you will need to peek at intervals to see when it is flat.
• ·         Your starting point for the top temperature to use will be about 10°C lower than that at which the original was slumped, normally.  The amount of time may need to be extended significantly. The reason for this is to avoid as much marking on the finished side as possible.
• ·         Shallow forms and thick pieces will flatten more quickly than others, so a lower temperature can be used.  You will still need to observe the progress of the flattening.
• ·         Angled shapes and deep forms will need more heat and time than the shallower ones. 
• ·         Thin pieces may require more time than thick pieces.
• ·         Tack fused pieces need more attention and slow rates of advance to compensate for the differences in thicknesses.

Separators

• ·         Kiln washed shelves are usually adequate for flattening.
• ·         Thinfire or Papyros are needed when flattening upside down to ease any sliding necessary.
• ·         Powdered kiln wash or aluminium hydrate can be dusted over the kiln washed shelf when it is felt the form will need to slide on the shelf while flattening.

It may be that after all this, you feel it is not worth it to flatten.  It certainly is worth the effort, if only to learn about the characteristics of the form and its behaviour in reversing the slump or drape.