The process

Burnishing

A process older than any coating, and one that works differently for exactly that reason: the colour is not applied, it is formed.

Brass consists of copper and zinc. During burnishing the surface is brought to react in a chemical bath. The topmost atomic layers of the metal convert into a thin layer of copper and zinc oxides. That layer is not an application but converted base material: it does not sit on the metal, it is the metal.

From this follows the key difference to everything that is sprayed, powdered or vapour deposited. An applied layer can detach, flake or pull back at an edge, because there is a boundary between two materials. With a conversion layer there is no such boundary. What ages on a burnished surface is the metal itself.

Comparison

Burnishing among other processes

Four routes to a dark metal surface, and what separates them.

BurnishingChemical conversion of the surface into an oxide layer. No application, no boundary, grinding and brushing remain visible. Possible on brass, copper and steel.

LacqueringA colour layer on the metal. Partly covers the texture, protects reliably, and is hard to repair invisibly once damaged.

Powder coatingA baked polymer layer. Robust and free in colour, but the metal underneath is no longer visible. We do not offer this process.

AnodisingAn electrochemically formed oxide layer, technically limited to aluminium. It is not an option for brass.

The point

Anyone who wants a surface where the metal still reads as metal will end up with a conversion layer. Anyone who needs an exactly defined colour across a thousand square metres is better served by a coating.

Control

How the tone is created

The tone depends on how far the conversion is taken. At the start there is a light, only slightly broken brass gold. The longer and more intensely the reaction runs, the darker the layer becomes: through warm brown, dark chocolate brown and grey brown to an almost black anthracite. Nine of those states are described as fixed finishes so they stay reproducible.

What cannot be controlled is the exact marking. The reaction does not run at the same speed everywhere; it follows the rolling pattern of the sheet, the preparation and the guidance by hand. The result is a living surface with clouding, not an even coat of paint. Two batches of the same finish share the character, not the image.

For design that means: the colour character belongs in the specification, the exact marking does not. Where large continuous areas are built, we agree in advance how elements are produced and sorted so the surface reads calmly as a whole.

See the surfaces

From the workshop

Handwork in Nürtingen

Two recordings from our workshop. Before the bath the brass is ground, afterwards it is rinsed. Both steps are done by hand, and both help decide how the finished surface looks.

Grinding. The preparation stays visible under the burnishing, which is why it decides the later marking.

Rinsing. After the bath the reaction is stopped and the surface cleaned before it dries.

Over the years

How burnished brass ages

IndoorsThe state stays almost unchanged for years, because the surface is exposed to neither weather nor rain.

Outdoors, unsealedThe surface keeps darkening and becomes calmer over time. At the Federal Constitutional Court in Karlsruhe that is exactly the intention, observable since 2008.

Outdoors, waxedThe development continues at a slower pace. Wax wears off and has to be renewed, but the state can be steered through care.

Outdoors, lacqueredThe colour stays at the state of delivery. A damaged lacquer layer cannot simply be reworked the way wax can.

Green patinaUnlike pure copper, brass rarely develops a closed green layer outdoors. The zinc content shifts the ageing towards brown and grey brown.

Design

Contact with other metals

Brass is nobler than aluminium, zinc and galvanised steel. When those metals come into direct contact with moisture present, the less noble metal is attacked. In practice that means a substructure and fixings in stainless steel, or separating, non-conductive layers.

For every other question of use, from the area through the fixing to the location, please ask us. We also say when burnished brass is not the right choice for a project.

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