Granulation (jewellery)
Granulation is a jewellery manufacturing technique whereby a surface is covered in spherules or granules of precious metal. The technique is thought to have its origins in Sumer about 5,000 years ago. In the first millennium B.C. the technique was used by Etruscans living in present-day Italy. Greek craftsmen also employed the technique, but it was the work coming from Etruria which became famous, in part due to the mysteries surrounding the process. [1]
There are three basic techniques that may be used to attach granules to a metal surface: hard soldering, fusing and colloidal soldering. The metals used in granulation are usually gold and/or silver alloys of high purity - alloys below 18 kt. gold and sterling silver being unsuitable. With each technique the process begins with the making of the granules themselves. The granules are usually made from the same material as the base to which they will be affixed. Very narrow fringes may be cut along the edge of a thin sheet of metal, a further cut producing small squares or rectangles of metal. After being melted into globules, these may be sorted into various sizes by the use of appropriate meshes. Another option is to use thin wire coiled around a mandrel, the coil then being cut into small rings - this creates even-sized granules when melted.
Hard soldering metal items together exploits the capillary action of solder with a lower melting point flowing between the items to be bonded. Soldering is routinely used by bench jewellers and is an ancient technique, but is applied with great difficulty to small metal grains. Extremely small pieces of solder are positioned close to the point of contact between the granule and the metal, and then heated to melting point. The necessity of repeating this process many hundreds of times renders the technique impractical and expensive, complicated by the possibility of dislodging granules already attached. An improvement to this method is filing the solder to powder size and mixing it with the flux. The work area is painted with tragacanth and the granules are bedded on this using a fine brush. Having dried, the work is sprinkled with the flux-solder powder and heated to melting point. Surplus solder will lodge in the gaps between the granules, an effect which cannot be avoided.
Fusing joins metals composed of the same alloy by the use of heat alone. A sheet of metal of thickness near the diameter of the granules ensures an even heat distribution. The granules are positioned using a diluted flux and fine brush, after which the sheet is fired in a reducing oven. At melting temperature the granules and sheet metal fuse, leaving no flux or solder. This technique was used by the Etruscans in the first millennium B.C. Modern techniques achieve fusing by using electric current and the process is known as fusion welding. The electric current passes through each granule, generating sufficient heat to weld the granules to the base. This method may be used on finished items, even those with set stones.
Colloidal or eutectic soldering was used by the Etruscans, and employs a colloidal mixture of tragacanth gum and copper salts. The mixture lowers the melting points of both granules and base, and causes the copper to diffuse into both at the point of contact, creating a strong metallic bond. The colloidal solder is painted onto the surface of the base, and the granules arranged on the wet solder using a fine brush, after which the piece is left to dry. It is then fired in a reducing kiln causing the tragacanth to burn off and release the copper salt. At the eutectic temperature (890 °C) the copper diffuses into the granules and base, joining them together.