Hi,

i built my cob oven 6 weeks ago and we have had a few firings and pizza nights and it works well- i love cob ovens- simple and rewarding in so many ways. My question is relating to the protective plaster coating. Can you put a lime plaster on top of an earthen plaster?

My insulation layer was clay slip and sawdust as suggested in kiko denzers book. Initially it seemed to flake when it was drying and i didn't want it to just blow off and fall apart so i applied an earthen plaster which was made from clay soil, sand and wheat paste and chopped up straw. I didn't want to wait til then oven dried out completely before i plastered because i had the anxiety of losing my insulating layer to the elements.
The plaster itself has held up well to firings and only hairline cracks are appearing in the plaster which is less then 1 inch thick. I would like a more protective coating which i feel a lime plaster would offer. I am not sure if i can combine the two types of plaster so i would love to hear anyones thoughts on what i can do to make a more waterproof coating. I do have a small roof over the oven but sometimes the rain blows in sideways and it gets exposed.

many thanks for your time and replies.

I've been mixing 2 scoops of mason's Lime into my batch mixture
this is at a point well before your finish coat situation.
Troweling it smooth seems to work well and after a bit of a rainy few weeks, the mixture seems to hold up to being almost completely exposed.
so far
If the plaster is on the outside, then I don't feel like the heat will have any effect.
what little water you get onto the outer surface shouldn't touch it either.
Granted, I haven't done a thin lime layer over clay, but there seems to be no significant shrinkage differences with what I have done

mixing lime into cob will make it a little bit caustic on bare hands and eyes
so be warned
just don't mix up the pure lime with your bare hands or anything and you'll be fine

Hi,

I am just wondering what your batch mixture is and what will your final plaster coat be?

I have been soaking some hydrated lime for 6 weeks, i have some sand, i have lots of clay.. what should the recipe for a reliable plaster coat be- considering i already have a plaster coat containing the clay, sand, straw and wheat paste (which is cracking more now)

I know that cob ovens crack and eventually might need rebuilding. I would like a plaster coat that is more aesthetic then the cracking up earthen plaster I have now.

many thanks