Blue plaster is a traditional plastering technique that produces a wall finish with a characteristic bluish-grey tint, historically created by mixing lime plaster with specific natural pigments, blue-grey sand, or additite materials such as wood ash or crusite.
History and composition
Blue plaster has a long history in European building traditions, particularly in the Low Countries, northern France, and parts of Germany. The bluish coloration was typically not achieved by adding a separate blue pigment but rather resulted from the choice of local aggregates and the chemical properties of the binder.
In many traditional recipes, the blue-grey tone came from using river sand or quarry sand with a naturally high content of dark minerals. In other cases, small amounts of charcoal, wood ash, or soot were mixed into lime mortar to produce the desired hue. Some regional variants incorporated natural earth pigments containing iron or manganese compounds, which produced a cool, blue-grey tone when combined with the white of slaked lime.
The plaster was typically applied in two or three coats: a coarse scratch coat to key into the masonry, a floating coat to level the surface, and a fine finishing coat that carried the distinctive colour. The finishing coat was often burnished with a steel trowel to create a smooth, slightly polished surface that enhanced the blue sheen.
Application in construction
Today, blue plaster is primarily encountered in heritage restoration and conservation projects. Plasterers working on listed buildings or historic properties may need to replicate the original blue plaster finish using traditional materials and methods. This involves sourcing appropriate lime putty or hydraulic lime, matching the original aggregate, and applying the material by hand with traditional tools.
Modern lime plaster suppliers offer pre-mixed coloured lime plasters that approximate traditional blue plaster finishes. These products simplify the process while maintaining breathability and compatibility with historic masonry. Builders should avoid using cement-based plasters on old walls, as their rigidity and low vapour permeability can trap moisture and damage the underlying structure.
Related terms
- Lime plaster
- Scratch coat
- Heritage restoration
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