Heat-Induced Colours on Metals

November 15, 2013

Polished mild steel heated with a blowtorch produces this dark spectrum of colour.

One of the most magical things about metals in general is their ability to transmit a wide range of colours under different conditions.  The most brilliant and enduring pigments of the pre-industrial painters’ palette come from oxides, acetates and carbonates of metal ores and minerals. And the metals themselves, either in pure or alloyed form, yield a stunning range of colours, particularly in response to heating.

When you purchase mild steel in sheet form, its potential to yield colour is already apparent. There are gradations from light and dark greys to subtle blue tones, and maybe a bit of reddish oxidation here and there. These colours are only a few microns deep on the surface – when you cut or grind the steel it immediately reveals a very shiny silvery surface, which gradually patinates to grey. But if you apply heat to the newly exposed shiny surface, you get a most wonderful spectrum of colours – like a rainbow, but in a darker register.

The colours are a surface response to specific temperatures to which the metal is subjected, and engineers use these colours as a guide to the different degrees of hardness achieved by heat-treating the steel. This is because structural changes are taking place within the fabric of the metal as well as on the surface in response to rising degrees of applied heat.

Harmonograph pendulums resting on the hearth after a day’s work creating colour with a blowtorch. The rod in the foreground has not yet been heated. I sectioned the pendulums into polished and unpolished stripes to create contrasts in texture.

My motivation for heating the steel is nothing like as sophisticated as that of an engineer toolmaker – I just love to watch the magical transformation take place before my eyes, and to try to manipulate the colours as an aesthetic exercise.

Pattern-welded steel billet with heat-induced colour

As you train the concentrated flame of the torch onto the steel, it gradually turns a straw colour as it begins to reach a temperature of around 230 degrees Centigrade; as the temperature climbs towards 270 degrees it cycles though brown and then into purples, and finally, at about 300 degrees it gets to a beautiful indigo shade. The colours continue to cycle and flow along the piece of steel as the heat travels through it – it is actually very difficult to control. If you continue heating beyond 300 degrees you lose the colours altogether, and the steel hovers at grey for a while before it begins to glow through the reds and thence to the searing yellows (at over 1.000 degrees centigrade) which is forge-welding temperature.

Heat- colours continue to patinate, even after rapid cooling with water – they can darken, or they can gradually lose the colour, because it is actually a very delicate and microscopically thin surface layer of crystallized material that refracts light to create the impression of colour. During the pattern-welding workshop I undertook with Owen Bush, we experimented with heat on one of the billets we made – the layered magnesium-rich and nickel-rich steels showed slightly different rates of colouring, producing some beautiful patterns.

Some of the most spectacular responses to heating can be seen on brass, which is an alloy of principally copper and zinc. When Ricky Lee Brawn, engineer and metalwork technician at Central Saint Martins, heated up my brass plate to anneal it ready for hammer-forming, we got the spectacular and sadly ephemeral results in the picture below. Colours as delicate and elusive as the surface of a soap-bubble!

Surface of annealed brass plate.

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