Often even in shallow rectangular moulds the sides pull in during the slump. To know what things to try to correct this effect, you need to understand why this effect is occurring. These two pieces show the effect in different ways.
This slump shows the interesting effect that the further up the piece you look, the greater the curvature. This relates to the greater amount of movement required by the glass to conform to the mould at the outer edges.
During the slump of a rectangle or square the whole shape of the glass sheet is changing. It is slightly stretching to form into the “hollow” of the mould, but it cannot stretch evenly all over, especially at the corners. If you think of the analogy of Draping a piece of cloth into a rectangular depression, you will find it wrinkles up at the corners if you smooth it at the sides. This indicates the material is attempting to overlap there as it does not a dart to take up the excess cloth.
This similar to what is happening to the glass sheet. It is relatively thicker at the corners than along the sides. Therefore, it does not slide down the mould at the corners as on the sides. It is simply thicker and is compressed by the movement of the glass at the sides.
The question is how to use that knowledge to avoid or minimise the dog boning during the slump. There are probably lots of methods, but three have occurred to me and others.
Add more material along the sides. This involves fusing a piece with shallow arcs rather than straight sides. This gives more material to counteract the dog boning effect when slumping a rectangle. The difficulty is getting the proportions of the arc correct in relation to the length of the sides. You also need to ensure the arcs on the sides are not so much larger than the mould that they slump over the edge. This means the whole piece will need to be cut smaller than the mould.
Remove material at the corners. This takes the opposite approach. To avoid the increased amount of glass at the corners, you remove some of it. That is, you round the corners of the pieces to be fused. How much you will need to round the corners is a matter of experience, but is a shorter learning curve than cutting the edges in an arc.
Reduce the temp and increase soak time. This approach requires less skill in cutting a shape. It relies on giving the glass time to relax into mould with a minimum of stretch. You need to find the lowest practical temperature at which to slump. This will be the temperature at which you can first see the deformation of the glass in the mould. Hold the temperature there for as long as it takes – possibly one or two hours. It is likely that you will still need some rounding of the corners of the glass, but only your experience will determine that, and if so how much.
Cold work the edges until straight. This can be done by hand or by machine.