Short answer
Fired brick colour comes mainly from iron and other metal oxides in the clay, changed by how hot and in what atmosphere the kiln fires it. Iron-bearing clay fired with plenty of oxygen turns red; the same clay fired in a reducing (oxygen-starved) atmosphere turns dark brown to black; lime-rich clays fire buff or cream. For one clay, hotter firing gives darker, denser bricks. Sand facings, engobes and glazes add further colour, and because the colour is fired in, it does not fade — though mortar, efflorescence and soiling change how a wall reads.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Efflorescence | White salt bloom, often in the first year ('new building bloom') | Salts, mostly from mortar and adjacent concrete, carried out by water | Keep brickwork dry during and after construction; dry-brush |
| Soiling and organic growth | Darkening, green algae on shaded faces | Pollution deposits and persistent damp | Gentle cleaning only; avoid abrasive methods that open the fired face |
Clays are mostly silica and alumina with varying metallic oxides, and the Brick Industry Association's manufacturing guidance names iron as the oxide with the greatest effect on colour. In an oxidising fire, iron-bearing clay forms red iron oxide and fires to reds and oranges whatever its raw colour. If the kiln is starved of oxygen near the end of firing — called flashing or reduction firing — the iron is reduced and the same clay goes dark brown, purple or black, often unevenly, giving flashed ends and faces. Calcium and magnesium oxides lighten and yellow the colour; lime-rich clays give the buff, cream and yellow stocks familiar in parts of England. Manganese is added to darken bricks to brown and grey-black.
For the same raw material and method, darker colours go with higher firing temperatures, lower water absorption and higher strength — a harder-fired brick is both darker and denser. Cooling rate also changes colour, and so does where a brick sat in the kiln: bricks set face-to-face are more uniform than those set cross-wise, because faces exposed to flame and gases colour differently. Manufacturers blend clays from different parts of the pit to control variation, but subsequent production runs still differ slightly. That is why bricks are ordered from one run for a job and mixed from several packs as they are laid, rather than used pack by pack.
Many bricks do not show their body colour on the face. Soft-mud and stock bricks are moulded in sanded moulds, giving a sand-faced texture whose colour is partly the sand's. Extruded wire-cut bricks usually have the smooth die skin removed and texture rolled or scratched on. Engobes — slips of fine clay or colourants — can be applied to the face and fired on to give colours the body clay could not; they are hard but not impervious. Glazed bricks carry a true glass surface in almost any colour. Texture matters for colour: a heavily textured face holds shadow and reads darker in raking light than a smooth brick of the same clay.
From a distance a wall's colour is a mix of brick and joint, and mortar can occupy a fifth of the face. Mortar colour comes mostly from its sand, which is why conservation guidance says that matching the sand usually matches the colour of historic mortar. White efflorescence — soluble salts brought out by water, usually from mortar and adjacent concrete — often appears in the first year as 'new building bloom' and shows most on dark bricks. Over decades, pollution and algae darken faces unevenly, particularly under sills and on shaded elevations. Aggressive cleaning can strip the fired skin and change colour permanently, so gentle methods are preferred.
| Factor | Direction of change | Why |
|---|---|---|
| Iron in the clay, oxidising fire | Towards red and orange | Iron forms red iron oxide |
| Same clay, reducing fire (flashing) | Towards dark brown, purple, black | Iron is reduced; often uneven across faces |
| Lime or magnesia in the clay | Towards buff, cream, yellow | Calcium and magnesium oxides lighten the fired body |
| Higher firing temperature (same clay) | Darker | More vitrification; denser, less absorbent body |
| Cooling rate and kiln position | Variation within a run | Faces meet different temperatures and gases |
| Sand face, engobe or glaze | Surface colour independent of body | Coating fired onto the face |
Why: Bricks laid pack by pack from different kiln positions or runs.
Fix: Order from one run and blend from at least three packs as you lay.
Why: New building bloom from mortar salts.
Fix: Keep work covered during construction; dry-brush bloom — it usually diminishes.
Why: Wrong sand or cement-rich mortar compared with the weathered original.
Fix: Match the sand and binder to analysis of the original and compare with weathered joints.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Of the oxides commonly found in clays, iron has the greatest effect on fired brick colour: iron-bearing clay fires red in an oxidising atmosphere and dark or black when fired in a reducing atmosphere (flashing).
Source: Technical Note 9: Manufacturing of Brick (December 2006)
FactModerate evidence
For a given raw material and manufacturing method, darker brick colours are associated with higher firing temperatures, lower absorption and higher compressive strength.
Caveat: The relationship does not hold across bricks made from different clays.
Source: Technical Note 9: Manufacturing of Brick (December 2006)
FactModerate evidence
Efflorescence is usually a white deposit of soluble sulphates and carbonates carried to the surface by water; on brickwork under a year old it is called new building bloom and is commonly linked to salts from mortar.
Source: Technical Note 23A: Efflorescence — Causes and Prevention (June 2019)
FactStrong evidence
Sand is the largest component of mortar and gives it its distinctive colour and texture, so matching the sand usually achieves a colour match for repointing.
Source: Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.