Lime plaster vs gypsum plaster

The main difference is how they set. Lime plaster hardens slowly by carbonation: it absorbs carbon dioxide from the air and turns back into calcium carbonate, a kind of limestone. Gypsum plaster sets quickly by hydration: water recombines with heated gypsum and it locks into a hard mass within minutes or hours.

That difference drives everything else. Lime is softer, more flexible and lets moisture pass through it, so it suits solid-walled period buildings. Gypsum is fast, hard and smooth, so it suits modern construction and dry interiors. In old houses, the general rule from heritage bodies is to repair like with like, which usually means lime.

How lime plaster works

Lime is made by burning limestone or chalk in a kiln to make quicklime, then slaking it with water. Non-hydraulic lime putty sets only by reacting with air, so it hardens from the surface inwards over weeks and keeps gaining strength for months. Hydraulic limes contain clay impurities and set partly with water, which makes them faster and stronger.

For internal walls and ceilings, plasterers traditionally used lime and sand in several coats, with animal hair mixed into the base coats to bind them. Goat hair and horse body hair were favoured. On ceilings, the first coat was pushed between riven oak or chestnut laths so that it curled over the back and formed "nibs" that held the ceiling up.

• Slow set. Each coat needs time before the next, and the work must not dry too fast.
• Flexible. Lime tolerates the small seasonal movement of old timber and masonry.
• Vapour open. Moisture can move through it and evaporate from the surface.

How gypsum plaster works

Gypsum is a soft mineral, calcium sulphate dihydrate. Heated to around 120 to 180°C, it loses most of its water and becomes plaster of Paris, named after the large gypsum deposits near Paris. Add water and the reaction runs back the other way, forming interlocking crystals. Pure plaster of Paris sets in minutes, so wall plasters contain retarders to give the plasterer working time.

Gypsum has a long history in decorative work because it sets fast, expands very slightly as it sets and picks up fine detail from a mould. That is why cast ornament, from Georgian enrichments to Victorian fibrous plaster, has so often been made with it. Through the twentieth century, gypsum wall plasters and plasterboard largely replaced lime for new buildings.

Lime breathes and moves with an old house. Gypsum is fast and hard, and happiest in a dry, modern one.

Breathability and old houses

Most houses built before about 1919 have solid walls with no cavity and no damp-proof membrane. They manage moisture by letting it soak in and dry out again. The Society for the Protection of Ancient Buildings has long argued that such buildings need to "breathe", and that sealing them with impervious materials traps damp.

Gypsum plaster is not well suited here. Most bagged gypsum plasters lose strength when they stay wet, and some contain water repellents. Gypsum is also rigid, so it can crack where it meets older lime work. Replacing lime and lath with gypsum and plasterboard in a listed building is unlikely to be acceptable, and may need listed building consent. Check with your local planning authority before changing materials.

When to use each

• Repairs to old solid walls and lath ceilings. Lime, matched to the existing work. Use a plasterer experienced in lime.
• Damp-prone walls in period buildings. Lime, and deal with the cause of the damp first. Gypsum will fail and may trap the moisture.
• New partitions, extensions and modern houses. Gypsum plaster or skimmed plasterboard. It is quick, smooth and ready to decorate within days.
• Cast ornament. Gypsum plaster has long been the material for casting, because it takes a crisp impression from a mould.
• Fixing ornament to a lime ceiling. Make sure the ceiling is sound first. A loose lime ceiling needs a specialist before anything is fixed to it.

Finishes matter as much as the plaster. Limewash, distemper and mineral paints keep lime walls breathable. Modern vinyl emulsions form a tighter film, which is fine on gypsum but can hold moisture in an old wall.

Where to go next

Our pieces are hand-cast to order in Shrewsbury and arrive as natural plaster, so they can be painted with ordinary emulsion once sealed. Our page on paintable plaster covers the basics, and our guide to heritage colour palettes helps if you are choosing finishes for an older house. For the longer story of the material, read our short history of plaster.