Oh, here is a great primer course about Lime Mortar, this is just Part 1/8, but the rest of the course is easily found: http://www.youtube.com/watch?v=7xBzrjKAsTs&feature=channel_video_title

So here's what I've tried so far, using hydrated lime: Materials: - Hydrated Lime - Fine grain sand/silica - Water Make Lime-putty: I made a bucket-full of lime-putty. I filled a 5-gallon bucket about half-full with water, then added enough lime to create a putty texture (cream-cheese is the consistency typically described).

I allowed the putty to settle for about 1 week. Determine sand-ratio: I then took 300mL graduated cylinder, and filled it with 100mL of water. I added enough sand to determine the amount of lime-putty needed. Basically, when the level of sand reaches the level of water, then you'll know the ratio of sand/lime-putty.

Create Mortar: Using the ratio found above, mix the sand and putty to create the mortar. Wet-stones: Before applying mortar to the stones, I doused them a bit with a garden hose to wet the surface. This is done to mitigate suction somehow.

Apply Mortar: I had to mash the mortar onto the stone to get it to stick nicely, so it wouldn't just fall off. I was quite surprised by this, and am not sure if my mortar mix is correct.

Result: After a couple days, my initial joints all cracked. But, my 2nd round of mortar joints held up a bit better (I think this is because I was more forceful in mashing). I haven't really tested these joints though, I'll have to make another "Mortar sandwich" to test with. In previous tests, my mortar sandwiches were easy to pry apart with my hands, and the mortar easily crumbled between my fingers. So, I'm doubtful at the effectiveness in my latest batch of mortar.

Wow, looks like there's a lot of tips, tricks and techniques when making lime mortar. Seems like a great choice - especially because of the yearly frost heaving you'll see every winter in Minnesota. I've never used the stuff myself but I'm interested in trying it out some day and I think I found some good links on the subject below. The short answer is that it depends somewhat on the type you buy so you should read what's on the bag if there is instructions. Outside of that (pretty obvious) advice, it sounds like you need to let the mortar set for about a day before putting a lot of weight on it (such as another layer of stone). To get a good answer on this subject requires a bit more information. For example, are you planning on mixing the lime mortar with cement or are you strictly using what's in the bag? What does the bag say on it for directions? What's the brand? And how high is your stone wall - a taller wall could require more time to set between stone layers.

Looks like a lot of people use a mostly lime mortar with a bit of cement mix added to help it get rigid faster. This site has a recipe with this technique as well as some useful information on lime mortar.
Exerpt: Add just enough water to the mixture so that it becomes like a thick batter in inconsistency. While mixing the lime mortar mix, use your concrete trowel to gauge the consistency. Add water slowly until the lime mortar mix is pliable enough to spread easily without sliding off the trowel......Although modern mortar mix uses water to set, lime mortar uses an entirely different process. Lime mortar mix takes a considerable amount of time to set because of a process that is called carbonation. While regular concrete or mortar mix hardens due to a reaction with the water, lime mortar makes hardens because of a reaction with carbon dioxide and moisture.

Others use the lime mortar mix as an additive to a traditional cement mixture in order to add flexibility and strength to what is otherwise just regular cement. This guy seems to have a good recipe and uses it for laying flagstone

One of the answers on this yahoo answers page actually has a lot of good details. But it seems like they're talking about the hydraulic type.

This site has a lot of great info on lime mortar and plenty of tools and even training DVDs. However, it also sounds like it's related to the hydraulic type of lime mortar. Could still be useful though.

One interesting tip I saw is that you should cover the stone with an insulating blanket after applying the mortar.