Gypsum or lime?

[samberzina]

Hello,

We have started hacking off some of the existing plaster in our Victorian terraced house and behind what looks to be a gypsum skim coat, we have found a layer of plaster which is a similar pink to the gypsum, however the consistency of it looks much more similar to lime, quite grainy. Is there any sure way to tell what exactly is it? Have attached some photos below for a reference. It might be worth noting that this is an external facing wall which appears to have had historic damp proof injection works carried out. Considering this, we expected to see gypsum plaster up to approx. half height of the wall, however it is the same pink-ish base coat of plaster all the way up to the ceiling level.



Any help / advice would be much appreciated, thank you!

 

[Lacan07]

The thicker yellow band looks like lime to me, can you see any hair? The thin top coat could be a gypsum skim.

You could get some vinegar and see if it fizzes, this doesn't always work though.

 

[RBailey]

It could also be a thick bonding coat.

As Lancan said do a fizz test. Just be careful to test only the stuff you want, if there is any lime or cross contamination it will still fizz, albeit to a lesser extent.

This is some tests I did recently:



Cheers,
Richard B.