Natural clay mortar and plaster help required please.

My house is approximately  1880 a natural stone construction built on soil.  I am at the first stages of experimenting with clay mortar and need to infill voids between the natural rocks of my house and also the same with the interior , then experiment with plastering outside walls and inside walls .
I believe the exterior walls long ago were plastered but has long since fallen away , only some small areas remain plastered on the exterior walls. It appears to be a very fragile mix , I,m not sure if this was done with pure clay soil or a clay soil with a small amount of cement added  the same is true for the areas on the interior walls, it is a fragile crumbly mix and can easily be picked off the wall,  the small stones in the mix appear to be factory uniform.
I think when the wonder material Portland cement arrived the locals here may have used it and added it to the clay ,  i have taken some of the internal wall plaster and ground it up and it has a rather acrid stench to it, i cannot be sure of this though , all i know is that it has far less clay content in it than stones , totally crumbles up.

The mortar between the wall rocks though appears to be genuine wild collected clay ranging from white to red interspersed with natural stone / rock fragments and fragments of slate ranging from 1mm up to 10 to 15mm in totally irregular shapes. This has a definite tough feeling but can be broken up with a good amount of pressure / hammering.

My question is with my limited range of sieve sizes { and knowledge ) what sizes and ratios should i mix the fine clay to stone / particle sizes to fill the voids between the wall stones.
My sieve sizes are :  10mm  6mm  3mm  1.2mm  0.6mm , and from the 0.6 the clay fines pass through.
I have meticulously taken samples of the collected clay soil from my forest ,   from the wall rocks void mortar and the wall plastering and sieved them all.  I have  made notes of the amounts of each sieve sample then drew a bar chart of each size and its volume in milliliters then calculated % for each size in the sample,  over this i plotted a curve as a guide to compare each sample,  so i,m making inroads, i think but i,m not sure what stone grain sizes should be chosen for the void mortar and the wall plastering .
I,m looking for a starting / reference point from someone who has done this please , can anyone point me to a clay experimental / success area .

I would expect Lime mortar to have been used.
Portland cement causes all sorts of problems with moisture.

I have seen videos by The Nito Project (associated with Canelo Project) on Youtube that may have helpful info for you, or at least provide another avenue of inquiry: https://www.youtube.com/c/TheNitoProject/videos

When people built a house in the days before shipping materials across country, they used the materials that were on site. They didn't have special sieves and didn't waste time sifting their way to perfection. The size of the void and the materials at hand will impact the approach to the stuffing, so to speak. The first pass will utilize particle sizes as large as possible to speed up the work. The last layers of plaster are the finest material and are often applied like paint.
What would help is some pictures of the type of stone work, size of gaps, and similar local building examples. If you give us an idea where you live, we can better understand what materials and effect you are after. If the building is structurally sound, all sorts of plaster effects are possible. We need more information about your project and the vernacular style of your region to help you Malcolm. I am sure you will hear more answers when readers see the current condition of the building and the style you would like to achieve. Sounds like a wonderful project!

Ok i will post up some pictures  shortly.  Yes agree the materials would have been very local sourced indeed , lugging heavy stones and clay long distances would have been inthinkable.  I have looked at builders sand and lookes at the fine and small stones from the clay here and builders sand has an amazing predictable regularity where as the natural stones from the clay bearing soil are completly irregular , perhaps perfection is not a good thing with builders sand , hmmm.

Modern mason's sand is carefully sieved to have zero bits of gravel, so that masons can lay tight mortar joints fast and not have to worry about the odd pebble causing issues with bedding. "All-purpose" sand from a store may have irregular sizes; I know when I used that in lime mortar for the scratch coat on my walls, there were stones that were too thick even for the 3/8" thick first layer. Never mind using it for a finish coat.

I have worked with removing old lime plaster from a 1910ish house, and they had either bad lime or not enough of it, because the wallpaper was the only thing holding it together in some places. Then there were cement patches where I had to destroy the lath to get the patch off...

For base coats between stones of an old wall, I think coarse sand with pebbles would be fine, and perhaps even for a final coat if you are not looking for a totally smooth surface or a thin "skim" coat.

We seem to live in a world now where if its not uniform or doesnt conform to ideals laid down by so called professional bodies it is disscredited or malinged as unsuitabe or un acceptable, i assume there was none of that prior to 80  or 100 years ago , the people just did what was required and necessary rather than have to conform to certain standards and codes. A whole different mind set ie doing with what you have.