Carpets and Staining
Some carpets will stain if you give them a filthy look. You must take this into consideration when fitting a carpet, or it could come as a nasty surprise if there is a soot fall which splashes out onto the carpet. Such spills and stains should be left strictly alone, for a professional cleaner to deal with, as your attempts to remove them could result in nasty, permanent stains.
In the event of a bird coming down the chimney and causing similar damage, some insurance policies will cover the cost of cleaning/replacement. Phone them before attempting to clean things up.
Staining on the Chimney Breast
The salts formed in the chimney (See ‘Chimney Chemistry’) don’t stay in the liner, but are the victims of a tug of war. They make the lining hygroscopic (ie the salts pull at the water at the liner flue interface) as does the stream of gas/air passing up the chimney. Normally the stream of air is strong enough to keep the liner comparatively dry, and there is not enough water to soak the salts further into the chimney structure.
The real problem starts when a once used Victorian chimney is closed off, and the steady stream of air stops. Instead the air from outside enters from the top, bearing with it a cargo of moisture. The salts pull at this, and gradually the salt solution soaks further and further between the bricks and out into the plaster on the chimney breast which can discolour badly if the acidic salt solution is contaminated by tars.
There is little real cure for this save the removal of all contaminated material, for example stripping the breast of plaster and replacing it with waterproof plaster. While not a permanent cure, it can effectively be if the problem takes long enough to work around the repair. This is why the repair has to be radical. Simply removing the stained portion and replastering will most likely soon show a ring of staining as the salts creep past the repair.
Chimney Chemistry
The binding agent in cement mortar is cement which is a calcium atom with two hydroxyl groups attached. It is written thus: Ca(OH)2.
Sulphur from the fuel burns in the fire to give sulphur dioxide (SO2), which reacts with water in the flue gas to give sulphuric acid (H2SO4). This reacts with the cement thus:
H2SO4 Ca(OH)2 > CaSO4 2H20 (ie acid plus base equals salt plus water)
The calcium hydroxide is turned into calcium sulphate, and similarly into calcium nitrate and calcium carbonate (chalk). These salt groups are bigger than the OH groups they replace, so they swell, and because they are weaker than the cement particles, the liner expands and softens, crumbling away. The whole stack actually gets taller, and where one side is cooled by the wind, it bends like a bimetallic strip in reverse (ie the colder side expands).
This chemical reaction can also lead to staining on the chimney breast.
