Help with identifying plaster type

[casper.slonina]

Hello everyone, it's my first post on this forum even though I was reading it as a guest for a long time.

We recently bought a terraced victorian house and started renovating one of the bedrooms. After educating myself a little about lime plaster, I decided tackle it myself as house is relatively small and I'd only replace it on external walls which are mostly susceptible to damp problems. I'm quite happy with the results so far - it's definitely far from perfect, but as an amateur I do think it's decent enough. I finished float coat, and next week I'll be doing the top coat. So far I mixed my own plaster in ratio of 1 : 2.5 of hydrated non hydraulic lime to sand.

I have two questions that I can't find satisfying answer online for.

1. Can top coat be made by simply mixing hydrated non hydraulic lime with fine sand in 1:1 ratio straight away? Or should I make a lime putty first by adding water to lime and then mix it with sand after 24h?

2. Internal solid walls are plastered with original black ash lime plaster, however with my lack of knowledge I can't identify if a finish coat is made of lime or gypseum? I would like to keep them as they are, just refresh it with a coat of either gypseum or lime top coat. My decision which one to choose would be based on what material is it. If it's gypseum, putting lime top coat wouldn't make sense as it's not breathable anyway, but if it's lime, I can finish whole room in lime top coat for more homogeneous finish. I'm gonna attach a photo, hopefully somebody could help me with it.

 

[LadyArowana]

There's a test for lime plaster using ordinary vinegar. Basically lime plaster will fizz a bit in vinegar, other sort won't. There's various YouTube videos showing what to expect if you search lime/ test with vinegar

 

[malcolm]

Your work is looking good. If the top coat is what you have in your hand it looks like lime. Gypsum is pink and doesn't have any stones in it. It's been painted over, could well be a modern waterproof emulsion unless you found it under wallpaper. In any case use lime finish plaster on a lime base.

1:1 is quite lime rich. I've used 1:2.5 lime to silver sand for finish plaster. I think adding more lime would add more shrinkage.

I used NHL 3.5 as that's what I had knocking around at the time but I must give hydrated lime a shot for some plastering as it's a lot cheaper. But I'll have to defer to other members as I've not done it yet.