Metal Working Techniques Part 7: Granulation

My first thought when I went to start this blog was that Granulation is magical and amazing. Now I realize that this is obviously an overstatement to most people, but understanding the process makes the sentiment correct for me. Granulation is a surface decoration technique that uses tiny granules (spheres) of  metal that are soldered, fused or eutectic soldered to the metal surface in patterns. The technique has been used for thousands of years and was probably invented by the Sumerians. Granulation was usually done in gold, although it is possible to do it with silver.

Gold serving Bowl with Pulviscolo Granulation ...

Gold serving Bowl with Pulviscolo Granulation Etruscan 7th century BCE (Photo credit: mharrsch)

OK, so what is the difference between soldering, fusing, and eutectic soldering?

Soldering actually involves adding a lower melting temperature metal alloy (solder) to the surface of a metal piece and melting the metal alloy to hold two pieces of metal together. A chemical solution called flux allows the solder to flow into the tiny gaps between the metal, fill them and join the pieces of metal together. This technique is difficult to use with complex granulation designs because not all of the granules will solder at the same time and if you repeat the process some of the granules usually fall off. You also usually wind up with large puddles of extra solder on the surface of the piece in order to make the granules stick.

Fusing involves using granules and a metal surface that are made of the same metal alloy. The alloy is usually either fine silver (99.9% pure silver) or gold with a purity of at least 18k. The granules are temporarily glued in place with a special diluted flux and then the piece is heated in an oven until the surface just starts to melt. When the piece comes out of the oven the granules are fused to the surface of the metal piece and the flux has burned away. Modernly this process can also be done using electricity, which causes the granules to fuse to the metal piece without additional heat.

Eutectic soldering uses a special combination of plant gum and copper salts to glue the granules to the metal piece. When the metal is heated, the copper salts lower the melting point of the granules and the metal piece that they are glued to. This causes them to fuse together just where the granules touch the metal surface. The result is a very sturdy but neatly connected design.

I hope that this blog gives you a basic understanding of the process of granulation.

Next time: Filigree

Gold pin with a Mistress of the Animals (potni...

Gold pin with a Mistress of the Animals (potnia theron). Gold with granulation, ca. 630 BC. (Photo credit: Wikipedia)

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