In this series:
Part 1 – The Story so far
Part 2 – Johannes Gutenberg and his museum
Part 3 – A new museum
Part 4 – The Commission
Part 5 – The Artist
Part 6 – The Decision
Part 7 – The Gates
One of the chosen few
Part 8 – The Process
Part 9 – Talkin’ ’bout a Revolution
Part 10 – Lost and Unlost
Part 11 – But where are the originals?
I had difficulty getting my head around the casting process.
I knew that Karl-Heinz Krause had used antique printing blocks from the Gutenberg Museum’s inventory to create the imprints for the bronze panels, but I couldn’t for the life of me understand the complete process.
Bernd Hettinger (pictured) of the KunstgussKastel art foundry in Mainz-Kastel was kind enough to spend an hour or so showing me (as in ”recreating-the-entire-process-apart-from-pouring-the-bronze” showing me) how the panels for the gates would have been made back in 1961.
And a HT to Dorothee Wenz for pointing me in his direction.
Karl-Heinz Krause started off with a box full of printing blocks that he’d chosen primarily for their shape and size.
“I was looking for balance and rhythm” he told me “And some of the blocks were so pleasing that I used them more than once.”
And in an aside ” And I had to duplicate some, because we didn’t actually have enough…”
These he set in a 1.6 metre by 1 metre plaster of paris base in a high-sided frame (called the “drag”) and filled with moulding sand with the profile that he wanted extending above the sides of the box. (Think of an iceberg with only part of the ice above water and you’ve pretty much got it.) The sand is tamped down hard so that it forms a solid mass.
Don’t think “sandcastle-solid” mass. Think “drying-concrete-solid-break-it-up-with-a-sharp-object” mass.
The sand in the drag is smoothed off to an almost polished finish and the sand and printing blocks are sprinkled with a fine talcum-like powder to make for easy separation.
Now a second frame (the “cope”) is set on top of the drag, the printing blocks and the surrounding sand in the drag are covered with a fine sand followed by moulding sand, tamped down hard.
Now you hold you breath and separate the cope from the drag, revealing a perfect female indentation – a legible mirror image – in the cope.
Now comes the tricky bit.
You take the female mirror image you’ve just made as the drag, place the cope on top, spread the talc powder over it, cover it with fine sand and then tamp down the moulding sand,
Hold your breath again, separate the two frames and – eureka! – you got perfect male and female images.
Which aren’t really much use, come to think of it, because they fit so perfectly that there’s no cavity for the bronze to flow into.
So you take the male image and remove just enough sand from the surface to correspond to the thickness of the panel that you want.
Then you make the pouring hole (“sprue”), the gating system that takes the molten bronze from the sprue to where it needs to go and the riser (which shows you when the mould cavity’s full)
Put the cope and the drag together again – you’ve now got a gap between them for the molten bronze to flow into – heat up the bronze to around 950ºC (depending on the alloys) and pour.
It looks like a biblical image of Hades
Let it cool, split the cope and the drag and you’ve got your panel.
Tidy up the excess flashings and pouring remnants and you’re finished.
More over here at Wikipedia
Well, actually, you’re not, because the bronze didn’t flow evenly throughout the mould, so you have to cut out the faulty bits, recast them and weld them in to the panels using a new and – for the time – highly experimental electric welding technique.
And THEN you have to weld the 18 castings together to form 6 panels measuring 1.6m wide by 3m high.
And then you have to get them to Mainz in time for the Grand Opening of the museum…..
Tomorrow: Talkin’ ’bout a revolution