Granulation (goldsmithing)
The granulation ( latin granule = "granules") is an ancient goldsmith's art, in the smallest gold beads to an ornament to be laid or to a surface and then soldered onto a gold ground so that they are interconnected only at their respective points of contact. Light and shadow create a strong plastic effect. The Etruscan granulations are world famous .
History of rediscovery
The Roman goldsmith Augusto Castellani , who lived in the middle of the 19th century, devoted a large part of his work to the rediscovery of the lost ancient technique of granulation, which was used in European goldsmithing until the early Middle Ages. In his work he used a powdered solder with which he attached the sand-like spheres to the substrate. Due to the capillary force , the liquid solder was drawn into the spaces between the spheres lying against one another and thus fused to form a hermetic surface.
In 1918, Marc Rosenberg published his book on the technical basis of the history of goldsmithing and, among other things, stimulated a new, broad search for the technique of granulating. Chemical tests on antique pieces of jewelery showed that the base metal and the balls each had the same fineness. Rosenberg concluded from this that the connection comes about through a kind of “chemical solder”. He assumed that when the balls were melted and afterglow in charcoal dust, they would be coated with gold carbide and thus the melting point would be reduced by around 160 ° C, which would enable them to be welded onto the carrier metal.
One of the first to produce granulations that could withstand comparison with their ancient models on a technical level was Johann Michael Wilm in Munich around 1920 . Elisabeth Treskow began producing granulations at a high level of design in Essen-Margarethenhöhe around 1930.
technology
To make the balls, fine gold snippets and charcoal dust are layered in a fireclay crucible and rinsed out in water after melting. This resulted in perfectly round gold balls. However, the prerequisite for success here is gold with a fineness of at least 900 / °°°.
Lower alloys such as 14 karat (= 585 / °°°) are granulated with copper salts. Copper sulfate , copper oxide , copper chloride or copper hydroxide are equally suitable for granulation. They are mixed with highly diluted organic glue (hide glue, isinglass, etc.). When melting, the copper salt is converted into metallic copper, which is alloyed with the gold and thus forms a “chemical solder”, which then enables the balls to adhere. The glue burns to carbon , which enables the reduction of the copper salt to metallic copper.
Such alloys can also be granulated with the help of diluted Fluoron , a commercially available flux for brazing heavy metals , or tragacanth . This creates a much smoother surface than granulating with copper sulfate.
Collections
A large number of antique jewelery with granulations is exhibited in the Staatliche Antikensammlungen , Section IV, in Munich.
Elisabeth Treskow's private collection, which includes both antique jewelry and work by her own hand, contains a number of extraordinary granulations. She bequeathed them to the Cologne Museum of Applied Arts , where most of them can be seen in the permanent exhibition.
literature
- Jochem Wolters, Granulation , in: RDK Labor (2014)
