A borax cone is a solid, moulded form of anhydrous borax (sodium tetraborate, Na₂B₄O₇) used by jewellers, goldsmiths, and silversmiths to prepare a working flux for soldering and metal-joining operations. One of the oldest and most enduring tools on the jewellery bench, the cone is rubbed in a small quantity of water against a roughened slate or ceramic dish — commonly called a borax slate or borax dish — to produce a milky, paste-like suspension known as borax slip or borax cream. This slip is brushed onto metal surfaces immediately before and during the application of heat, where it melts to form a glassy, protective layer that prevents oxidation and assists the flow of solder.

Composition and Chemistry

Anhydrous borax is the dehydrated form of the naturally occurring mineral borax (tincalconite or kernite-derived sodium tetraborate decahydrate, Na₂B₄O₇·10H₂O). In cone form, the material has been fused and cast, driving off the water of crystallisation to yield a dense, glassy solid. When heated on metal, the borax melts at approximately 741 °C, forming a fluid, transparent glass that wets the metal surface, dissolves metallic oxides, and creates a barrier against atmospheric oxygen. This dual action — oxide dissolution and oxygen exclusion — is what makes borax an effective flux for the precious metals (gold, silver, and platinum-group alloys) routinely worked in the jewellery trade.

Use at the Bench

Preparation is straightforward. A few drops of clean water are placed in the borax dish, and the tip or side of the cone is rubbed in a circular motion against the dish's roughened surface until a smooth, creamy slip of consistent viscosity is produced. The jeweller controls the concentration by adjusting the quantity of water and the duration of rubbing — a thicker slip for heavier joints, a thinner wash for delicate filigree or fine-gauge wire. The slip is applied with a small brush to the joint area and to the solder pallion before the torch is introduced.

During initial heating, the water in the slip evaporates and the borax passes through a characteristic bubbling, swelling stage as residual moisture escapes. Experienced bench workers learn to advance the flame slowly through this stage to avoid displacing solder pieces. Once fully fused, the flux settles to a clear, glassy film that flows with the molten solder, guiding it into the joint by capillary action.

Advantages Over Proprietary Fluxes

Commercial paste and liquid fluxes — many of them fluoride-bearing or formulated with active chemical agents — have become common in production workshops, yet the borax cone retains a loyal following among traditional craftspeople and those trained in European goldsmithing traditions. Its principal advantages are simplicity, economy, and the degree of control it affords: the jeweller can vary slip consistency to suit the task, and the material contains no additives that might contaminate alloys or interfere with subsequent finishing. Borax is also non-toxic in normal bench use, though adequate ventilation remains advisable when working with any flux at soldering temperatures.

For platinum and high-melting-point alloys, borax alone is generally insufficient — specialist fluoride-based or boric acid fluxes are preferred — but for gold and silver work across the full range of karatages, the borax cone remains entirely fit for purpose.

The Borax Dish

The cone is inseparable from its companion tool, the borax dish (also called a borax slate or grinding dish). Traditional examples are made from unglazed fired clay or natural slate, both of which provide the slight abrasive texture necessary to abrade the cone and suspend particles in the water. Circular ceramic dishes with a roughened central well are the most common form encountered in contemporary workshops. The dish requires periodic cleaning to remove the glassy residue of fired borax, which can be chipped away or dissolved in warm water.

Historical Context

Borax has been used as a flux in metalworking for many centuries, with documented use in European goldsmithing at least from the medieval period. The cone form — convenient, portable, and slow to deteriorate — became the standard presentation for bench use and has changed little in character over that time. Its persistence in an era of sophisticated proprietary products speaks to the practical wisdom embedded in traditional craft practice.