I've been considering the idea of using lime putty to finish the joints on the inside of the roof. This is for multiple reasons. 1) Lime-based substances are breathable, so if my joints on top aren't completely watertight (and I'm pretty sure I am fallible!) and some moisture gets past them, it can evaporate out through the lime.
2) A putty-like compound will be easier to work with on the ceiling. It is more likely to stick in place than a mortar. I will just have to wear safety glasses and be careful, though, because hydrated lime is extremely basic (pH of 12).
3) I've been curious about lime because up until the beginning of the 20th century all mortars were primarily lime-based. (See this post) As well, Internet research has yielded confusing information on the merits of hydrated vs. hydraulic lime. So far, I've determined that hydrated lime does not cure, it sets by absorbing carbon dioxide from the air. This process is slow because the chemical reaction is happening between the substance and its environment. Hydraulic limes react with themselves, sort of like cement. It still takes longer to harden than cement but it is faster than hydrated lime and will harden under water. The Romans made their lime hydraulic by adding pozzolan. Hydrated lime is often added to cement mortars to improve their workability, and this has led some people to believe that hydrated lime needs cement to harden. This is not true, it just takes a long time.
I have a lot bags of hydrated lime that I didn't use last year for my mortar. Just simply mixing lime and water makes a putty, but the trick is that the putty gets better (I'm not sure what 'better' means yet) as it matures. Apparently it can go for years, although the minimum is 24 hours. This time, I've gone for the minimum. Maybe I'll do more tests with the same batch after waiting a bit.
I decided to do three mixes. One just pure putty, one with fine sharp sand, and one with grog (crushed brick, see this post). Grog is supposed to also have a pozzolanic effect, turning hydrated lime into hydraulic lime. That was another trick of the Romans.
One website had said that lime putty has the consistency of cream cheese. They are exactly right.
When mixing I noticed the absorbency of the grog. I needed to add more water to that mix to keep it workable.
Then it was just a matter of applying it to the joints. It is a skill I will have to work on, but I think I can clean them up nicely. I just have to wipe down the brick after doing the joint. Another thing I've read is that a wet knife will smooth out the putty nicely. I didn't have too much success but I am hopeful. I will check back in a few days to see if it is hardening.