SHRINKING AND STRETCHING THEORY
1 Thinner pastry = stretched
Thickness is directly proportional to area because the volume of the pastry stays the same. By rolling it the molecules are displaced downwards and outwards. This increases area and decreases thickness: 75mm x 5mm becomes 105mm x 3mm.
2 Thinner steel = stretched
Here, we’ve done exactly the same thing using a hammer and mild steel. The strip of steel was 1.15mm thick and 10.15mm wide. After a minute’s hammering with a slightly crowned planishing hammer, it’s 1.00mm thick and 10.95mm wide.
3 From flat to crown
This headlamp bowl has a lot of threedimensional shape. It originated from a flat sheet of steel, probably stamped in one process in a press. This shape is called ‘crown’ in metal-shaping terms. Because it’s so pronounced, it’s ‘high crown’.
4 Raising and sinking
This sort of shape – used for helmets, etc – would traditionally have been raised by repeatedly heating and hammering the steel externally onto a stake from the centre outwards. Sinking is the same, but you hammer it internally into a hollow.
5 Introducing shrinking
In jam tart fabrication, the base of the bowl shape is left unstretched. As you can see here, though, there’s too much pastry around the edges – shown by the puckering, or tucks. It’s pretty obvious that the area around the sides needs to be shrunk.
6 Dealing with puckers
To lose the puckers, you push on the tucks’ peaks so they collapse into themselves, resulting in thicker but smoother sides. Essentially that’s exactly what you do with metal. It’s easier with pastry than with mild steel, but the principle is the same. 7 Mild steel tucks You can use the same process to shrink steel. Here, we created a pucker in the edge of a flat offcut [A], then used a hammer on a steel dolly to hammer the tuck down onto itself [B]. You can see from curve this created in the sheet [C] that the edge has been slightly thickened and thus shrunk. This process isn’t creating folds but is hammering the metal into itself, rearranging the molecules.
8 Stretched/shrunk flanges
If you fold a flange along the straight edge of a piece of steel sheet, there are no issues. If you fold one along a curved edge, however, the flange will be too long or too short, causing the sheet to distort. The flanges need to be stretched or shrunk accordingly to make the sheet flat again. Shrinking or stretching a flange on the straight edge of a flat sheet can be used to give the sheet curvature.