Published on 20/06/2024 by Conservationist
Look for the thickness and texture. Original lime plaster is often thicker than modern gypsum and has a more 'hand-finished' look. If you can see small white specks (lime inclusions) or hair (often horse or goat hair used as a binder), it's almost certainly a traditional lime mix.
Gently tap the wall. Lime plaster on lath (common in ceilings and partition walls) will have a distinct hollow sound but feels substantial. If the wall sounds very thin and 'drum-like', it might be modern plasterboard. If it sounds completely solid but looks like it's been 'skimmed', it might be a modern gypsum overlay on older lime.
Conservationists sometimes use a small sample and a weak acid solution. Lime (calcium carbonate) will fizz when it reacts with acid. This is a definitive way to distinguish it from gypsum, which does not react in the same way.
Before you start any renovation work, knowing what's on your walls is vital. Applying modern paint or plaster over original lime can trigger damp issues almost immediately. If you've discovered lime in your property, it's a sign that the building was designed to breathe, and any future work should respect that heritage.
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