Thermal Improvement of Amber

by Wieslaw Gierłowski

Correcting of the external appearance of ready-made products and works of art made of amber through heating dates back to ancient times. The method is inspired by natural phenomena. The amber which basked on the beaches and dunes became clear and obtained an increasingly intense hue.

Amber nuggets were thermally clarified and tinted in the Roman production centre in Aquilea, and in the millennia that followed in the workshops of the amber artists from the towns by the Baltic Sea, still unrivalled in their artistic ingenuity and master craftsmanship. Each master had his own recipe for improving nature in the desired direction.  However, these interventions were always comparatively mild and done through the use of the slow heating of amber in liquids (usually oils) or loose materials (sand or table salt).

It was only in the 2nd half of the 20th century that a tool appeared, which was as effective as it was risky. This tool is the pressure furnace filled with inert gas: nitrogen or argon. In the air-tight chamber of the furnace it is possible to quickly change the amber's bubbly structure into a material which is clear but unfortunately has lost its natural golden colour. We get a cold watery tint which is far removed from tradition and customers' expectations.

The gas in the furnace's chamber (called the autoclave) penetrates the amber's structure at a pressure of 300 atmospheres and at a roasting temperature of up to 300 degrees Celsius, altering not only its appearance but also its naturally delicate and pleasant scent.

However, you do obtain the clarity and uniformity of the material you want, and even the possibility of a second roasting with oxygen, which allows you to give a fashionable colour to the surface and produce the so-called scales, i.e. delicate, shiny cracks inside the gemstones.

Raw material, with the weathered layer, is processed in the autoclaves. When viewed through a clarified layer of amber (for instance at the bottom of cabochons), the weathered layer roasted in nitrogen has a greenish tint. There are cases of saturating the weathered layer with blue colouring which yields the effect of an intense green colour when viewed through the yellowish amber. Not every seller is ready to label this effect as artificial.

The vast majority of Polish amber product manufacturers do not improve amber in autoclaves, feeling that this sort of intervention goes too far. The unquestionable benefits such as: good clarity, the permanent binding of the amber layers in the so-called splice (layering material), increasing the hardness and decreasing the cleavage of the amber, are accompanied by detrimental effects: discolouring and deterioration of scent. These manufacturers use the old methods of roasting in liquids, sand, salt or even in furnaces with the access of air, through slow heating and subsequent cooling. The lesser degree of clarification yielded by this method is offset by the retaining of the natural colour in the entire structure of the amber nuggets.