Oiling natural plaster

Hello,
I insulated the stone wall of my son's bedroom with hemp and lime. Because he wanted a clean and smooth finish, I finished it with a plaster consisting of one part clay and 2/3 part powdered marble plaster. Now, I believe I need to oil it . This is my son's room, where he sleeps. So I'm very reluctant to use the obvious choice of linseed oil. My husband suggested that I use a black soap paste, as used in tadelak but I'm worried about humidity collecting on the surface, when he has always chosen to sleep on that wall, where he is next to a window. I remember reading somewhere about using a flour paste. I'm not sure if that would be breathable or not. Do any of you have suggestions of breathable, only-mildly odoured ways to finish an earthen plaster? Could I just ust olive oil? Or sunflower, or some other oil? I'd appreciate any and all advice offered.
Cordially,
Tiffaney

Earth builders that I have worked with recommend a wheat flour or casein wash. These will prevent dusting and help hold the surface together and not have any appreciable effect on breathability. You could also try a lime wash and then if that isn't good enough you can follow it with one of the other treatments.

Oiling earth plaster is not something I am familiar with or have heard of.

I understand wanting to protect the earth plaster.

I like Phil`s suggestion.

Have you thought of using milk paint?

Here are a couple of threads that might be of interest:

https://permies.com/t/215413/art/Milk-paint-fine-art

https://permies.com/t/235117/art/oil-paint-stink-risking-health

Hi Tiffaney,
I'm not a natural building expert, but I do read a lot. I have actually never heard of an oil finish on a hemp/lime wall. It sounds like a hempcrete mixture, and all the things I have read, suggest simply using a lime or clay plaster and that is it. I'm definitely going to do some more research, but I'm wondering if the oiling is necessary for that particular type of wall?

While I have never used the lime hemp finish, it sounds at least as durable as the fine clay mud finishes that I have mixed and applied here in New Mexico. Since your walls, humidity and other variables are unique, it might be useful to make some sample wall surfaces to try different materials. For example, to test finishes in my environmental conditions, I have taken adobe blocks, troweled on the plaster wall layers, then tried different surface treatments. To do this, replicate the finishing layers then experiment with the ideas mentioned above. Olive oil will probably smell rancid over time. Fat-free milk will not smell. White flour paste without the wheat germ will not smell. See what damage your son could do to the surface of the trial surfaces.
For a final finish, I have spritzed water-based carnauba wax on surfaces with success. I have also rubbed whipped car wax onto walls with beautiful results. Once applied, I use a light-weight electric rotary car polisher to get a super shiny finish.
Good luck with your project Tiiffaney!

I follow a group of craftsmen out of Latvia and they do some amazing work with timberframing and other natural building products. There is a video of the company owner building a wooden house online. At around 21 minutes in, he mixes up the clay/lime/hemp(?) plaster to finish the walls. I don't believe it requires any additional oiling or finishing agent.

https://youtu.be/RV7pmE4MC-I?si=snFsJi3k0X4No23Q