My house 180 yrs old, built of misc stone, some very hard some very soft plus bits of brick - I have heard it called 'random rubble'.  About 18" thick sometimes 2'. There is no plaster and the walls are very uneven - part of the character. The original material between the stones varies greatly sometimes pinkish with little white lumps, sometimes dark grey with black lumps ( I have wondered if it contains ash and tiny bits of coal!) Generally the original material looks pretty good but with a tendency to be crumbly but if I take too much out I'd have no walls left!

Problem is more with various past repairs in what looks like a plain sand/cement mix but done at different times cos different colours and sometimes very sandy and when you try to paint it just keeps crumbling. Tried sealer but still too crumbly. Elsewhere the cement mix very hard but has come away. So I have hacked off all past repairs which looked insecure and now want to make good, retaining the random look of the wall.

Big question: What mix, and should I put some lime in it? Or would it be even better to try to make up the sort of mix the original builders might have used. (I live in the country - I have heard about cow pats bing used and I'm not above collecting some from the field just down the road if anyone knows the recipe!

Incidentally I am talking about interior surfaces, external is either rendered and pebbledash or random hard stone with hard pointing which seems quite good and not obviously in need of attention.

Would appreciate some guidance please

Yoggo

Might be worth a read while waiting for JD...

http://www.bdma.org.uk/index.aspx?pid=131

This site is worth looking at;

http://www.mikewye.co.uk/

and this;

http://www.stastier.co.uk/


and this;

http://www.lime-green.co.uk/index.php?Pid=1

Many thanks to you all some helpful info - including the lager mix for the lunch break.

Yoggo

Responding to Mark S:

Just to let you know Mike Wye is not too far for a visit unless anyone knows of somewhere in Somerset where I am so I’ll take some photos and samples. Rydale looks like Scotland so that definitely is too far.

I'm not sure what pozzelan is - from little bits of info from Google search I get the impression it could be pumice or some other soft stuff from volcanic activity?

As to the interior surface – it appears as if whatever mortar was used to build the wall was just very roughly towelled over or brushed with a wet brush and then painted – a very higgledy piggeldy surface but the good bits are all part of the charm in a rural building! If only some of it, and the repairs in cement, had not fallen off.

The outside the rough cast seems quite hard and for all I can tell it may have cement in it, on some external corners it’s cracked but otherwise ok so I used a white exterior grade silicone seal and painted over it and its doing fine. I know that some of the upper storey walls are cob (mud + straw) so its essential to keep the wet out. fortunately the probs are on ground floor otherwise I'd be really worried

Yoggo

Oh yes, pozzlan.

Lime sets by a process of carbonation, modern limes tend to be very pure (industrial manufacture) and they can take a long time to set,  in the past lime was made from local materials (theres an old kiln at the end of our lane) and had impurities in it, soil, clay, whatever.  Those impurities can help the lime set (im not sure of the exact mechanism but its true, my very pure lime plaster took longer to set than that with bits in).  You can buy a volcanic pozzelan (at huge prices) but can also use powdered brick, ash, coal etc.  

Mike Wye has a very good reputation, and there are several lime specialists down your way, not so many up here.

most plasterers used to modern gypsum dont have a clue when it comes to lime.  (esp if you are using hair as a binder)  try it yourself.  Mike Wye runs courses I think ?

or try spab - they run lime courses - with a building of the sort you have it would be worth joining anyway.

best of luck

Mark