Nice example of salt contamination on a chimney breast.
Have you ever noticed discoloured patches which often appear damp on the face of your chimney If so, this could be caused by salt contamination.
Salt contamination within a chimney occurs due to the long term effects burning of fossil fuels (coal and wood) in an open fire. When fossil fuels are burnt, contaminates are released into chimney flue where they migrate into the brickwork.
The long term effects can result in extensive contamination of the masonry where these salts aggressively attack the brickwork. Often coupled with water ingress these salts can move within the chimney breast and are deposited within the surface plaster where they tend to give the appearance of a damp, discoloured greasy residue. These salts can be hygroscopic meaning they have the ability to absorb moisture from the atmosphere, therefore their presence within the plaster causes the plaster to become damp simply by attracting airborne moisture.
You can often see the effects of salt contamination throughout the year as these damp patches becomes more visible in the colder wetter months when humidity levels are high and reduce in the warm summer months as humidity levels lower. This is the effect of the contaminate salts absorbing moisture from the air. The effects may also be increased where chimney flues have been capped absent from ventilation.