saltpetre

[skye]

Anyone have any advice on what to do about those troublesome light brown patches on the walls that are a result of saltpetre so I'm told.  There's no apparent damp currently so is it enough just to isolate them with some kind of sealer and paint over?

[Musicmonkey<ADDRESS><STRONG>Musicmonkey<STRONG><ADDRESS>]

Skye,

I think that the occurance of saltpetre is a sign of a previous damp patch that has since dried.  Has something been done to resolve the damp that caused the saltpetre?  If not, it seems it will probably recur.  I don't know of any treatment for the saltpetre itself, sorry but if you use a sealer you may cause more problems elsewhere.

[Opel Fruit<P><BR>Opel Fruit, Dept. 53<P>]

We get this up to about 2 feet up on the inside of some walls, from time to time. We have tried various humidité/saltpetre sealing products from Rubson and Sika with very limited success. The Thompson stuff ain't effective either, though to be fair, it isn't for fluffy walls or odd stains! It seems there is little that can be done, short of re-treating and hoping. It would be a shame to Placo over it.

To the best of our knowledge, there are no damp problems (rising or penetrating) and as far as we can tell, no history of this. Having said that, we cannot verify for the 400 years of the walls' life!

It may be worth trying the products mentioned above, they are not difficult or obnoxious to use, but like anything you apply with a brush or roller in France, they are f.expensive.

And, if you DO find a "solution", DO let us know.

[David M]

I think I'd be tempted to clean it, with a non-residue soap/detergent and then paint it with a breathable paint.  And see what happens.

During the last 18 months of our renovation, we've encountered 4, possilbly 5 different types of stains on our walls.  They come in 3 textures and 4 colours.

On the lower parts of the stone walls in the house we have effervescent plaster.  That I'm certain is saltpetre.  In the outbuildings we have dark granular stains near the floor and a very light beige discolourating near the ceilings.

Back in the house on the top floor near the tops of the walls we have light brown patches (as you describe) and other areas that look like fossilised tree roots.

The cause of all of this we believe is different types of damp (humidité as the locals say with a smile).  18 months ago the walls measured at over 85% humidity.  Drilling a hole brought out a stone slurry.  The causes we are certain were bad land drainage (cured), rising damp (cured with gel injection) and a leaking roof (temporary repairs and a new roof underway)

The effervescing plaster in the house we have had to remove, it was no longer adhering to the walls.  We'll give the walls another 6 months, test for humidity and replaster if we are below 10% humidity.

The effervescing plater and the light beige stains in the outbuilds, we treated with products containing "piolite" a year ago.  It's all held fine and there don't appear to be any problems, we only treated the affected areas, leaving the walls other places to breath.  But we are reluctant to use the product indoors because it really does prevent the walls from breathing.

The dark granular stains on the old limestone walls of the outbuilding which were also treated with piolite (3 generous coats) have returned after a year. We think it may be a fungus.

Similar external areas we treated with a fungicide in the early spring all look perfect.

The fossilised tree roots, were diagnosed as a fungus due to damp coming into the wall from above.  The leaking roof was temparily repaired.  The wall dried, the stain quite litterly brushed off with light sanding, and the plaster is intact and still adhering well to the wall.

So to come back to your query.  The light brown stain hasn't changed in 18 months. The paster sounds sound;but  I don't think it's a result of saltpetre, but I do think it's a humidity stain.  I'm going to treat it with a fungicide and paint it with a non sealing paint (no vinyl) and just see what happens

The point about all this story is that there seem to be a whole variety of dicolourations (mineral and organic) encouraged by humidity. And as Skye says you need to cure the cause... and the only treatment for saltpetre leeching from plaster, is to remove the plaster...

so paint it and wait.

David

[skye]

Thanks for all this information - very helpful. My light brown patches are high up the walls and I suspect may have been due to the leaky roof (now repaired). The architect who told me they were saltpetre says it has something to do with animal urine used at one time in the mortar and will likely recur periodically, but I'll try the  cleaning and painting first and see what happens. Incidentally - how do you test for humidity in the walls? Is  a gadget required?- I'm afraid I'm quite new to all this, having just bought the house last week.

Skye

[skye]

Thanks for all this information - very helpful. My light brown patches are high up the walls and I suspect may have been due to the leaky roof (now repaired). The architect who told me they were saltpetre says it has something to do with animal urine used at one time in the mortar and will likely recur periodically, but I'll try the  cleaning and painting first and see what happens. Incidentally - how do you test for humidity in the walls? Is  a gadget required?- I'm afraid I'm quite new to all this, having just bought the house last week.

Skye