Lime plaster - really daft question

[Jen210]

Hi Everyone, I'm new both to this site (which is fantastic - the discussions pages have answered loads of my questions!) and to period properties, so please be gentle!

We bought a sandstone cottage about a year ago (built about 1820) which had been renovated by a man who can only be described as a complete idiot. If I tell you he wired up the pond pump using standard indoor electric cable with no waterproof casing :shock: (just ran it across the lawn) you get the picture!

The house has been replastered inside and repointed outside but we've discovered that we have quite a bad damp problem. The interior plaster is starting to bubble and the damp surveyor has said we have rising damp. I've read a lot on this site about damp and we've got plans to tackle it but I have a question (and this is the daft bit): how do I tell if the plaster / mortar currently on my house is lime or the standard stuff? I don't expect for a minute that it is lime but to be completely honest, I wouldn't know the difference! Any tips for spotting it?

Told you it was daft! Any tips gratefully received (particularly with regard to getting it all off if it's not lime!)

Thanks!

Jen

 

[glayzell]

lime will probably have hair in and be a whit/off white colour. gypsum will usually be a shade of brown and a bonding coat will be as well.

 

[David Q]

Lime plaster tends to be soft, white and a bit crumbly (at least in my house!). However I have never seen lime plaster bubble - that seems to be confined to gypsum. If the house was built in 1820 it would have been lime plaster, and only work after WW2 would be otherwise. So unless your house has been completely replastered (unlikely) it is lime. However it is also fairly common for repair work to have been done in gypsum, this is easily visible if wallpaper is removed, but difficult to see if paper is present. Try to examine plaster around your house (for example ceilings will be visible), most of what you see will be lime, then any different parts will start to stand out.

 

[skier-hughes]

pop a bit of the plaster in vinegar, lime will fizz.

 

[Craig]

Always worth remembering that a common "improvement" is to hack off the lime, inject a DPC and replaster with cement so you may find that you have two bands - cement below and lime above (after about 3' high).

I find vinegar doesnt always do that noticeably with old lime?

 

[MdB]

Cement will also fizz in vineger. I tried it a week or so ago and couldn't tell the difference in reaction between lime and cement. Might have been my samples though.

 

[skier-hughes]

Can't remember who recommended it here for me when I asked the question some time back, but it worked for me. Cement didn't fizz and lime did.

Maybe just an extra to help ascertain the truth.

Maybe the cement had been put on top of lime and had lime in/on it?

Or was one of those wonder mixes of cement and lime :roll:

Or vinegar is an old wives tale

Graham

 

[MdB]

The cement I tried was from a hard standing outside (30 years old max) and could have been polluted with lots of stuff, doubt it was lime though. Cement is alkali so the fizzing didn't surprise me.

 

[Carol]

Hello Jen,

I am not a building expert but your description of the plaster starting to bubble off rings bells! Our house, mediaeval timber framed was 'done up' in 1979 by a nutter (maybe the same one!) and has a layer of gypsum skim plaster over all the walls and some of the ceilings. This 'bubbles off' leaving the lime below when it is a bit damp underneath. We have found that we can lime plaster over the resulting hole satisfactorily. Obviously it would be preferable to carefully scrape the entire skim off and replaster with lime. But very expensive!

 

[Jen210]

Hi Everyone, Thanks for all your tips (sorry for the delay in the thanks - I've been working all the odd shifts over Easter that no-one else wants!). At least I now vaguely know what I'm looking for! The interior has been, I think, skimmed over with 'new' stuff, so I'll have to have a look what's underneath. Husband will be pleased - he loves taking things apart!

Thanks again,

Jen

 

[NT]

Lime plaster is normally white, though it can be pink. Gypsum is normally pink, so colour alone will ID lime in most cases.

Nearly all modernish plaster is gypsum, and nearly all historic (prewar) is lime.


NT

 

[Penners]

pop a bit of the plaster in vinegar, lime will fizz.

As MdB says above, his experiment showed that this isn't always the case. Fortunately, a chemist replied on that thread (apologies to chemist - can't remember your name), as follows:

"...the fizz is caused by the carbon dioxide being given off from the substance. The difference in the cements "fizzing" may be due to the different types of cement. Some cements are hydrated and have undergone a chemical process to drive off the carbon dioxide. Also over time old cement may absorb Carbon dioxide from the atmosphere where the new stuff might not have not had long enough to do so or possibly because of the process it has been through in manufacture may not have the capability of absorbing it. Lime mortar hardens by absorbing carbon dioxide, so will give it off when put into the acid (in your case the vinegar)."

How much of this applies to gypsum plaster, I'm not sure. Personally I would rely on NT's colour test, plus the fact that lime is softer than gypsum.

 

[NT]

Just to complicate matters, cement contains free lime. It is this lime that causes the skin irritation.


NT

 

[Penners]

When did you last buy a bag of cement? Believe me, there's nothing free about it.