Old brick is soft. We have old brick. We are repainting, so any pointing that should be done needs to be done now. Because the brick is so soft, we cannot use standard, "Type N" Mortar. It won't give and when the brick expands and contracts or takes on water, the mortar will hold fast and brick will spall. So, we need a more historically appropriate mortar, like "Type O" or even "Type K"
"Type N" mortar (i.e. what Quikrete sells as Mortar) should consist of six cups of sand, one cup of lime and one cup of Portland. A "Type O" mortar should consist of 2 1/4 to 3 times as many cups of sand as portland and lime. "Type O" should have 1.25-2.5 times as much lime as portland. "Type K" should have 2.5-4 times as much lime as cement.
What I did was take 2 cups of "type N" mortar and added 1 cup of lime and 3 cups of sand (of a fairly coarse and thus similar to original variety) to the mortar. Then, to toughen it up just a tad, I add another 1/2 cup of portland, or perhaps a bit less. So I end up with:
3/4 cup portland
1 1/4 cups lime
4 1/2 cups of sand
After doing more research, we've found that you can use lime in much greater quantities if it is finer. Not only that, but it's both easier to work with AND plenty tough. We also found that we were using WAY TOO MUCH PORTLAND. Fortunately, it was just shallow pointing and protected from water. The key thing to remember is no more than 20% portland to portland+lime.
Here's the recipe I used much later when replacing bricks:
1 part Type N Mortar
2 1/2 parts Lime
5 parts sand
We also added a teaspoon of putty colored dye. This makes the mortar almost invisible next to many older mortars. You can use a little less lime or a little less sand just watch that ratio of portland to lime.
More portland adds strength, but loses flexibility. There's a delicate balance. Too hard, and you damage your brick or worse. Too soft and you'll be doing the whole process again soon. I probably could have skipped the additional portland but I fear that it would take far too long for this relatively weak "type K" mortar to set up and we need to paint soon.
I'm pre-mixing the dry ingredients first, and keeping them in a left over kitty litter bucket with a lid. Then the painter can take out what he needs, add water to that, slake, and then go to town.
I did a test and with the larger, browner sand and the lime, this looks almost identical to what I have in there from 100 years ago. Not bad. Of course, the proof is in the pudding. We'll see how it looks prior to paint and after.
If you try to do this yourself, you'll want to read up on technique and get a few tools, but from what I've seen, getting the recipe down is probably the toughest part.
Below are pictures of the mortar only a few hours after application. The image above was of the same mortar mix, but which has cured for 24 hours.