How do I match new mortar to the existing colour?

The commonest reason a repointing patch stands out is that it was judged wet. A mortar that looks like a perfect match on the trowel dries several shades lighter and often a different hue entirely, and by the time that becomes obvious the wall is done. Everything about matching mortar comes back to making test samples and waiting.

Step by step

1. Take a sample of the existing mortar.A piece the size of a thumbnail from a sheltered joint, not from a weathered face. Break it open — the inside is the true colour, and the outside is decades of dirt and algae.
2. Look at what the sand in it is doing.Crush a bit between your fingers. You will usually see the sand grains clearly: their colour, their size and how well graded they are. That is the thing you are trying to reproduce.
3. Take the sample to a merchant and try their sands.Builders merchants stock several sands and they vary regionally and even between deliveries. Look at them dry, then damp. A yellow building sand, a red building sand and a silver sand give three completely different mortars from identical cement.
4. Make test batches, more than one.Three or four small batches with different sands, and if necessary different ratios, each mixed properly and gauged the same way. Label them. This step is what separates a match from a guess.
5. Put the tests on the actual wall.Point a few joints in an inconspicuous area with each candidate, or make small pats and stand them against the wall. A colour judged against a bench indoors tells you almost nothing.
6. Wait until they are completely dry.Days, not hours, and longer in cool weather. Mortar lightens dramatically as it dries and it keeps lightening after it feels dry. Judging early is the single commonest error.
7. Look at them wet as well.Walls are wet a great deal of the time in the UK. A match that works dry and is glaringly obvious after rain is only half a match. Splash the test panel and look again.
8. Adjust with sand before you reach for pigment.Blending two sands gets you most of the way and behaves predictably. Pigment is a last resort: it is easy to overdose, hard to reproduce exactly between batches, and some pigments fade.
9. Consider the binder as well.A grey cement mortar can never match a soft buff lime mortar however you adjust the sand — the binder itself is a different colour. On a historic wall, matching means using lime, not tinting cement to imitate it.
10. Match the joint width and profile too.A narrow crisp joint and a wide flush one read as different colours at a distance even in identical mortar, because the ratio of mortar to brick in the field of view changes. Copy the original joint dimensions.
11. Mix enough for the whole visible area.Colour drifts between batches, particularly as a sand heap dries out. Gauge strictly with the same bucket, keep the sand covered, and try to finish a whole elevation or a whole visible panel from consistent material.
12. Accept that new work looks new.Even a good match is a fresh surface next to a weathered one. It closes up considerably over two or three years. Do not chase the last ten per cent with pigment — you will usually make it worse.

Tips

• Ready-mixed tubbed mortars come in a range of colours matched to common historic mortars, and for a small repair they can save an afternoon of test batches.
• Photograph your test panel next to the existing wall in flat daylight. It is easier to judge a difference in a photograph than standing in front of it.
• If a whole elevation is being repointed, a slightly imperfect match matters far less, because there is no adjacent original to compare it to.
• Where the match genuinely matters — a listed building or a prominent frontage — a laboratory mortar analysis identifies the binder, the sand and the proportions from a small sample.

Common mistakes

• Judging the colour while the mortar is still wet — It dries very much lighter, so a match made wet is always too dark. This single error is responsible for most obviously patched pointing.
• Trying to correct the colour with cement — Changing the cement content changes the strength, which matters far more than the colour. Adjust the sand instead and leave the ratio alone.
• Matching to the dirty outer face of the old mortar — You end up making a mortar the colour of a hundred years of soot. Break the sample open and match to the clean interior.