Lime putty sits at the heart of every good traditional plaster finish. Whether you are patching a section of wattle and daub, laying a new base coat on a stone wall, or working up a fine skim for a Georgian room, the quality of your putty decides how the plaster behaves under the trowel and how it cures over the decades that follow. Get it right and the material feels butter-smooth, hangs on the hawk without slumping, and dries to that soft, chalky whiteness that only lime can give. Get it wrong and you will fight shrinkage, cracking and dust for weeks. The good news is that preparing lime putty is a slow, simple craft, and once you understand what is happening in the tub, it becomes second nature.

What Lime Putty Actually Is

Lime putty is the result of slaking quicklime (calcium oxide) in an excess of water. The reaction is vigorous and exothermic — it generates serious heat — and it produces calcium hydroxide in a thick, creamy suspension. That suspension is your putty. Unlike bagged hydrated lime, which has been slaked industrially and dried, putty is kept wet from the moment of slaking. This matters enormously, because calcium hydroxide continues to dissolve and refine in the presence of water. The particles get smaller, the texture gets creamier, and the finished plaster becomes more workable and more durable. Time, in other words, is an ingredient.

For most restoration work you will buy putty ready-slaked from a supplier and matured for at least three months. But if you are slaking your own — perhaps from a lump of quicklime for a specific job — the principles below apply equally.

Slaking Your Own Quicklime

If you are starting from quicklime, safety comes first. Wear goggles, gloves, a dust mask and long sleeves, and work outdoors or in a well-ventilated space. Never add water to lime; always add lime to water.

• Choose a clean plastic tub or lime pit, and half-fill it with water. Allow roughly three parts water to one part quicklime by volume.
• Add the lime in small batches, standing upwind. The mixture will hiss, steam and bubble violently. Stir gently with a long-handled paddle.
• Keep adding lime until the mixture reaches a thick cream consistency and the reaction quietens.
• Cover the tub and leave it to work for at least 24 hours, stirring occasionally.

Once the violent phase has passed, pour the putty through a coarse sieve to catch unburnt stones, grit and over-burnt cores. These inclusions are the enemy of a fine finish — a single unslaked particle will pop on the wall months later.

Storing and Maturing the Putty

Maturing is where good putty becomes excellent putty. Store it in a sealed tub or dedicated pit under a permanent layer of water at least 25mm deep. That water barrier stops the calcium hydroxide reacting with carbon dioxide in the air and forming weak crusts of carbonate on the surface.

Minimum maturing is three months for general plastering. For fine internal finishes — gauged stuff, skimming coats, decorative work — aim for six to twelve months, and the longer the better. Victorian plasterers often worked with putty that had sat for years. Keep the tub frost-free, because freezing damages the particle structure, and check the water level monthly. If a skin forms, skim it off and discard it; the putty underneath is perfectly sound.

Testing Consistency Before You Mix

Before adding sand, assess your putty. Drain off the standing water, then stir thoroughly. Good putty should look like thick double cream and hold a soft peak on the trowel. If it pours like milk, it is too wet; if it stands in stiff lumps, it is too dry.

• Too wet: leave it to stand in an open tub, or pour off the clear water that rises to the top.
• Too dry: add clean water a little at a time and stir until the texture loosens.
• Gritty: push it through a fine sieve before use, especially for finishing coats.

A quick check with a handful rubbed between gloved fingers tells you more than any measurement. You want a smooth, slightly slippery feel with no grain.

Mixing for Base Coats and Finishing Coats

With mature putty ready, you can build your plaster. For a typical base coat, mix roughly two and a half to three parts well-graded sharp sand to one part putty by volume. Add a handful of animal hair for reinforcement on lath or wattle. For a finer float coat, move to about two parts sand to one part putty, using a finer sand. For the final skim or decorative finish, work at one part sand to one part putty, or use neat putty gauged with a little fine aggregate.

Mix thoroughly and let the plaster stand for twenty minutes before use, so the sand and lime can settle into each other. Then knock it up once more and it is ready for the wall.

Common Faults and How to Avoid Them

• Popping: usually unslaked particles. Sieve your putty thoroughly and never rush the slaking stage.
• Cracking: often too much water or too much lime in the mix. Keep your ratios consistent and build coats thinly.
• Slow set: cold, damp conditions and thick coats. Work in mild weather and keep coats under 15mm where possible.
• Chalking surface: putty that has carbonated under a dry crust. Keep it stored wet and use it fresh.

Preparing lime putty rewards patience more than any other part of plastering. Give it time, keep it wet, keep it clean, and it will give you a finish that breathes with the building and softens beautifully with age.