Game, Set and 'Match' - Colour Matching Lime Mortar

I write this as Wimbledon plays on the television and thought the most tenuous connection between the tournament and this blog's subject matter - colour 'matching' lime mortar - just about warranted the use of the title. Probably.

Sometimes it's necessary to match the colour of existing lime mortar with the new mortar being used in a repair. The extent to which you match the colour of the mortar largely depends on how important it is to have a repair that is a true like-for-like replacement. Repairs to Grade I listed properties may warrant having the mortar matched professionally to assess what aggregates (sands) were used in the original mortar so that the same materials and proportions can be used again for the repair. Doing this will ensure that the fresh mortar closely matches the original in colour, texture and strength. However, for most of the repairs I come across, mortar which dries to an off-white colour is usually a perfectly acceptable option as most properties were built with this colour mortar in the first place.

Using a blend of different pigments to match mortar colours

But imagine a listed property that was built using soft handmade bricks and lime mortar, but was subsequently repointed in cement. The strength of the cement compared to the original bricks and lime is causing some of the bricks to begin to spall (the face of the brick breaks off). Given that the brickwork elsewhere may be at risk of the same damage, one solution would be to carefully remove all the cement pointing and re-point in lime mortar. Another would be to deal with the immediate problem through a localised, sympathetic repair to the damaged area, which in this case was the best option for the client.

In this scenario, making a repair to the brickwork using an off-white mortar would make the repair stand out like a beacon in sharp contrast to the surrounding bricks. A white grid amid a sea of olive/grey cement. No conservationist would want to make a cement repair to brickwork that was originally laid with lime mortar (particularly when it is seen to be causing damage) and so this is where selecting a different colour lime mortar comes into play. Whilst it's possible to mix a weak cement mortar mix (Type K mortar), choosing a lime mortar to match the original is likely to be the better option.

The colour of a mortar is determined by the sand it contains and there is a wide range to choose from - yellow sand, red sand, sharp sand, wareham washed, black grit and more. To save you having to mix these sands yourself, most conservation building material outlets sell bags of Natural Hydraulic Lime with a range of colours to choose from. These can be mixed on site when needed.

However, if you want to use a non-hydraulic lime, you must either add the sand to lime putty yourself (and know which sand to add and in what proportions) or buy a standard off-white ready mixed mortar and colour it with pigment.

Using pigment is an effective way to colour lime mortar

There are plenty of pigment colours to choose from and they can be mixed together to give a vast range of additional colours and tones. I channelled my inner scientist and found a bit of experimentation came in handy. I wanted to know how to create (and consistently re-create) a range of base colours but also, for a particular job I was working on, to colour match a lime mortar to cement mortar. (Conservationists want there to be a visible story when it comes to repairs to listed properties - to be able to see repairs, additions, alterations etc. I wanted to create a colour that almost matched the colour of the cement but was visibly different).

Experimentation with pigments and lime mortar

My experimentation involved using a small range of pigments - I chose five: Natural ochre, raw sienna, raw umber, burnt sienna and brown - and mixing them in different amounts and combinations. Some pigment colours are very strong and it really doesn't take much at all to colour the lime (it pays to buy a scale that measures in micrograms if you're using small amounts of mortar to experiment with). Others are weaker and so it requires more to give a stronger colour. It takes some trial and error, but doesn't take long to understand which pigments give the better results. It's important to carefully weigh and record the sample of mortar and the amount of pigment you add to ensure that the colour can be accurately replicated at scale on site.

I've now got a small range of colours to choose from and know how to replicate them consistently and I'll be using one of my creations on a job next week.

#listedproperty #buildingconservation #periodproperty