Why Mix by Hand?

Not every lime job needs a mortar mixer, a gang of three and a tarpaulin full of sand. Most of the work I do on old buildings — repointing a chimney stack, bedding a couple of replacement bricks, filling a deep void where a lintel has been swapped out — involves a bucket or two of mortar at most. For that, hand mixing on a board is quicker than setting up machinery, easier to control, and gives you a far better feel for what you are making. When you mix lime mortar by hand you notice the moment the sand starts to take up the putty, you feel when it is short of water, and you learn what a good mix is like through your gloves. That knowledge is worth more than any spec sheet.

The method below is for small repairs using non-hydraulic lime putty and sharp sand. It is not a recipe for large-scale work — for a full elevation you want a forced-action mixer and a consistent gauge — but it is the same principle in miniature, and it will see you through the sort of jobs that make up most historic building restoration.

What You Need on the Board

Keep the set-up simple and everything within reach, because lime mortar starts to stiffen as you work it. You will need:

• A clean spot board — a sheet of ply works well, damped down so it does not suck water from the mix.
• Lime putty, straight from the tub. Non-hydraulic putty stores under water and should be a smooth, thick cream, not lumpy.
• Sharp, well-graded sand. Washed sharp sand or a good concreting sand with a spread of particle sizes. Avoid soft, rounded building sand, which gives a weak, over-fat mortar.
• Clean water in a small can or bottle, plus a spray bottle for fine adjustment.
• A gauging trowel, a shovel or hoe for the bulk of the turning, and a rubber bucket.
• Gloves and eye protection. Lime putty is alkaline and will dry out and crack your skin; it stings badly in a cut.

Getting the Gauge Right

For most historic pointing and bedding work, a gauge of one part lime putty to two and a half parts sharp sand by volume is a sensible starting point. For pointing joints that will be finished and rubbed back, go slightly richer at one to two. For backing or deep filling where the mortar is not seen, one to three is fine.

Measure by volume, not by weight, and use the same scoop each time — a cut-down bucket or a large plant pot makes a reliable gauge. Consistency matters more than absolute accuracy. If you measure three sand to one putty on Monday and two and a half to one on Tuesday, your repairs will look and behave differently, and the difference will show in a year or two.

Mixing: Putty First, Water Last

This is the bit people get wrong. Lime putty already contains all the water it needs, so the sand should be brought to the putty, not the other way round. Tip your measured putty onto the damp board and spread it out. Add the sand in two or three stages, turning it in with the trowel and then working through it with the hoe or shovel.

Expect it to look dry and crumbly at first — that is normal. Keep turning and chopping, folding the outside into the middle, until the colour is uniform and no streaks of pure putty remain. Only then assess the water. If it is genuinely stiff and unworkable, add a splash from the can, no more than a tablespoon at a time, and work it in fully before adding more. Many mixes need no added water at all.

A word on sand condition: if your sand has been sitting out in the rain, drain it and let it dry a little, or you will drown the mix and end up with a weak, shrinky mortar that is miserable to use.

Testing the Mix

A good hand-mixed lime mortar should hold its shape on the trowel without slumping, and should spread under the trowel with a slight suction rather than sliding about. Take a small ball in your gloved hand and squeeze it. It should hold together, feel buttery, and leave a faint sheen on your palm without shedding water. If it crumbles, it is too dry. If water runs out, it is too wet, and you should add a little more sand and putty in gauge rather than trying to dry it out.

Once it is right, cover it with a damp cloth and leave it to stand for fifteen to thirty minutes, then knock it up again briefly. Lime mortar benefits from this rest — it fattens, becomes more cohesive and is far nicer to use. If you are not using it the same day, cover it properly and it will keep for several days in cool weather, though you may need a drop of water and a re-mix before use.

Working Clean and Working Warm

Use the mortar within a couple of hours of the final mix, and do not be tempted to add water to a batch that has started to set — you will only weaken it. Wash your tools at the end of the day in clean water; dried lime is abrasive and sets hard on trowels and buckets. Frost is the enemy of fresh lime, so cover work overnight in cold snaps, and keep new mortar damp for the first few days. Mixed and used with care, a hand-gauged putty mortar will match the surrounding historic fabric beautifully, and it will still be doing its job long after the bucket has rusted through.