Joints of Nom-Uniform Colour

Visualisation

After the grout has hardened, the joint pattern displays different shades of colour in different areas, particularly with materials of an intense colour.

Defect assignment

The commercialisation of grouts with a wide range of colours adds value to the aesthetic result of a ceramic tiling. However, the more vivid the colour, the greater the difficulties in obtaining a uniform colour in the entire wall tiling or flooring.

The chromophores present in a grout composition are of varying nature, solubility, and behaviour in the presence of water. The manufacturer needs to be aware of their compatibility with the requirable adhesion and technical characteristics of the joint once this has matured, but the manufacturer also needs to ensure that the colour furnished does not mean an added difficulty in the final cleaning of the wall tiling or flooring. The black or anthracite grey joint mortars obtained with micronised coal may be recalled, with the resulting impossibility of cleaning a tile when graphite particles entered the micropores of the tile (for example, the polished porcelain tile of some years ago).

However, in addition, the manufacturer needs to anticipate the possibly lower solubility of those colorants, so that the colour will not vary excessively as a function not just of the quantity of mixing water, but also of the loss of that water through different causes (evaporation, absorption or suction by joint sides or bottom, effect of the cleaning water, etc.). This is the major factor in the lack of uniformity in joint colour.

With grouts of intense colour, the tile fixer should, theoretically, take into account all the factors that might alter the quantity of water present in each and every phase of the joint maturing process, which is practically impossible. However, the grouting should at least be done under favourable environmental conditions, if possible without direct exposure to sunlight, ensuring that sufficient time has passed since the tile installation so that the residual moisture of the bonding material or of the tiles themselves will not adversely affect the achievement of colour uniformity.

A difference in water absorption capacity among the tiles making up a ceramic system that needs to be grouted with a material of the same colour may constitute a further problem, because the water present during the maturing of the joint mortar will vary in different areas of the tiling, causing colour differences. This defect may also occur if ambient humidity is high.

Re-pointing particular joint surfaces (for example, to eliminate bubbles and craters) may also cause differences in shades. An incomplete or deficient grout mixture will produce the same defect by areas.

Obvious differences in colour of some particular part of the wall tiling or flooring are assignable to point aggressions or to the presence of foreign materials contaminating the joint. Some cement-based grouts introduce iron oxide into the aggregate, giving rise to a yellowish coloration that may vary among different areas of the wall tiling or flooring. White grouts should never be cleaned with esparto, since that contaminates the joint with a yellow dye.

Finally, to be noted is the change of shade or even the discoloration that some materials containing reactive resins undergo with aging, by action of the sunlight, or by the continued chemical attack involved in permanent immersion in slightly alkaline or acid water (the case of grouts in swimming pool basins and surrounds).

Prevention

Since this defect is the result of multiple factors, it is recommended at least to observe the following:

• Always observe the manufacturer’s instructions in regard to the mixing and application of the grout
• Control the cleanness and humidity of the grout joints before applying the material
• Wet the edges of very absorbing ceramic tiles, especially when the tiling includes tiles with a different water absorption capacity
• Avoid grouting under adverse weather conditions and direct exposure to sunlight
• Protect the tiling from fast and non-uniform drying
• Include chemical resistance where it is needed
• Avoid the use of RG materials that change colour by the action of sunlight