Granulation comes from the Latin “granulum” or granum” meaning “grain”. It is a decorative process involving the application of small, usually spherical granules to a base object by fusion weld bonding. The base can be sheet, wire, tube or even other granules. The contact surfaces of the granule and base are brought to a liquid state by heating, allowing them to join, and then the work piece is allowed to cool and solidify the joint.
Heating the granules and base excites the atoms which then want to “travel” and make what is known as a “contact alloy”. Remove the heat and the contact alloy gradually recrystalizes into a metallurgical bond, the fusion weld. There is not a clearly demarked line between the two metals, the transition being gradual. This bond will not be the strongest ever made, but perfectly adequate for the purpose intended, i.e. to keep the granules attached to the surface of a jewelry item.
If you are using pure metal(s), atoms of one will penetrate the other to form the bond. If the same pure metal is used for both granules and base, surface liquation, after heating the work piece close to the melt temperature, will result in bonding. If different pure metals are used, (example fine silver base and fine gold granules), heating to get surface liquation of the lower temperature component will result in an “alloy” bond without heating to the higher temperature.
One of the most common karat gold alloys used for granulation purposes is 22K yellow. Hoover & Strong has now designed the popular 22K yellow that it too can be successfully granulated. It is available in wire, sheet and casting grain forms. So, to delve into this further, the process of making 22K yellow granules is outlined below.
This is the general name given to the process of making your granules. Usually wire is cut into small pieces that will eventually form the granules. The gauge of wire and piece size will depend upon the size of granule you want to make. There are numerous ways to do this, a few of which are outlined below.
Take a graphite sheet and place the cut pieces on this, making sure that none of them touch – if they do they will join together making a much bigger granule, maybe too big. Next you need to apply heat, either by torch or by using an oven. The pieces need to be heated above 1890ºF, the flow temperature of the alloy. When molten, surface tension makes liquid metal naturally form a shape that has the smallest surface area containing the maximum volume of metal– this shape is a sphere. Once this has occurred, allow the metal to cool and solidify and you have your granules.
There is the potential with this process to get some flat surfaces due to gravity when solidifying, especially with larger granules – the liquid metal just weighs too much for the surface tension to keep a sphere. One trick is to cut small depressions in the graphite sheet with a round burr– this helps the solidifying granule keep its spherical shape. Another alternative, if using a torch, is to angle the board over water so that the granules roll off into the water.
An alternative way to make granules is to take a suitable container – stainless steel is great – and put a layer of graphite or charcoal powder on the bottom. Place the cut wire pieces on top of this layer – again making sure none touch. Cover these with another layer of powder and again place cut pieces on top. This can be repeated several times, but youʼll have to get a feel of how many for your own process and equipment capabilities. Once done, place the container in an oven, set above the flow temperature of the alloy – for 22K yellow, above 1890ºF, I suggest 1950ºF to 2000ºF – for 15-20 minutes. Again, when the alloy melts, spheres will naturally form. Remove and allow to cool. Some people dump the contents into a water bath once the melt point has been passed – this is ok if it works for you. The carbon powder does not resist the spheroidising process and allows great quality granules to be formed.
A further advantage of this method is that the alloy will not oxidize due to the reducing nature of the surrounding graphite powder. Again this is not so much a problem with Hoover & Strong 22K yellow, but if lower karats are used it is a phenomenon that may occur.
A third and often well used method is much simpler but requires a greater skill level. Jewelers often just take the wire and melt it with a torch, allowing the liquid alloy to drop into water to cool. This works quite well, but you need good torch control and also be aware of the drop height to keep the particles as spherical as possible.
Once youʼve got your granules, you can size sort them to your requirements and merrily granulate away. I will not go into the actual joining techniques here, the subject is far too broad and there are numerous methods jewelers use to do this which depend on metals used, work piece type etc. Most involve coating the granules with a flux/glue mix to attach them to the base and prevent oxidation when heated to make the fusion weld.
For further information why not find a course dealing specifically with granulation in jewelry design, such as the one offered by Metalwerx School for Jewelry and the Metal Arts, Waltham MA. www.metalwerx.com 1-781-891-3854. Contact Karen Christians. Copyright® 2006 Stewart Grice
Further reading on this technique:
“The Jewelerʼs Directory of Decorative Finishes” by Jinks McGrath
“Jewelry Concepts and Technology” by Oppi Untract
“Complete Metalsmith” by Tim McCreight