Mortar dye

[Mathew Newton]

I know what you mean!

I followed the 'rules' from the pavingexpert.com site, namely never have four corners meeting and try to minimise long runs of unbroken lines. I ended up printing/cutting out the shapes in paper and laying them out on a plan - this worked well (apart from when the cat took an interest) and then it was just a case of copying it out.

Even then, once I'd thought it was good to go, the guy at the builder's merchant immediately spotted a four-corner-meet - clearly I'd got to the stage where I couldn't see the woods for the trees! Of course, getting rid of that 'error' without causing another one was tricky but we managed it in the end!

Mathew

[Aidan Karley]

That would make them "pigments", not "dyes" in most uses. For practical purposes, that equates to "inorganic and insoluble" not "organic and soluble". Brown pigment required, reasonably stable on a million-year sun-&-rain basis of testing? I'd look at variations on burnt umber, limonite and the like. Do plenty of batch testing, and specify in great detail to the supplier that you want a hydrous iron oxide and nothing else. Ask them for the chemical analysis - should be around about 60~70% (by weight) iron. Only thing I'd be a little cautions about is that I'm not sure of how umber would react to the high alkalinity of a lime mix. Like I said, batch test.

[Aidan Karley]

"Lamp black" is a posh name for soot, made as a pigment. 100 years ago it was a major product for making inks, paints, etc. These days people have long since realised that (1) it's a hand-made product, therefore expensive compared to vat-made dyestuffs; and (2) it contains non-carbon compounds like polycyclic aromatic hydrocarbon, which are potentially carcinogenic. So, whatever is in your modern mortar dye probably isn't "lamp black", which is pretty much the definitive "black man looking for a black cat in a coal cellar at midnight" version of "black". You might be able to still get lamp black if you specify it. Also, your 100-year-old mortar will have lost appreciable slightly-soluble salts over the century, which will still be around in your new stuff. Look at the stalactites growing down out of an old brick-and-mortar bridge - that "stal" material came out of the mortar, and is more or less white, and it's still present in your new "black" mortar.

[Andy Wade]

The modern equivalent "carbon black" is still extensively used as a filler and pigment in plastics and rubbers.

[Stuart Noble]

All manner of pigments are available from art suppliers if you want to get nerdy about it, but oxides are the most intense, and are widely available from BMs.