I have marked in chalk the area that I want to heat on the horn. Both sides of the horn show be heated and whilst the heat will radiate out from this area as long as the heat is directed at the marked area this will not cause any problem.
This is that particular part of the horn squeezed up and the bulge can be seen where the heated part of the horn has distorted with the pressure. Notice that there is no distortion on any other part of the horn and no cracks on the back of the horn that would have occurred if the horn had been heated in the wrong place or not sufficiently.
This shows the second area of heating and compressing and the differences in the shape of the horn in these three pictures can be seen. Each time the horn is squeezed it must be allowed to cool down before being removed from the cramp. Notice the pieces of glasspaper between the jaws and the horn to stop it slipping when providing the pressure.
(Part 3)
After squeezing the core hole and cutting the joint side of the horn
one is able to get a better idea of where the horn should be
squeezed to bring it to the correct shape. For this I use an
electric paint stripper for the heating and a sash cramp to apply
the pressure. I am unable to give the correct degree of heat to be
applied, I was taught that it should be sufficient so that one could
touch the horn but not to hold it. However the man who told me this
had hard hands after a lifetime of manual work, so I am afraid this
is something that comes with experience.