I got plenty of local clay and I see it can be used with perlite for a heat riser. What ratio of perlite to clay should I use. What are some of the natural equivalents of perlite? Should anything else be added or just water, clay n perlite?
I suspect there isn't a clear simple answer to your question. Not all clays are equal. But first, perlite is a natural product, just the process is not natural, it's just puffed granite. There are other products like light weight lava pebbles, vermiculite which is puffed quartz. The size of the grains of perlite or any of the others is important, smaller is better and consistency is key. If you want to make it a little easier to get consistent quick results you can add up to about 15% portland cement, or better "quick set" cement, but quick set is harder to find as just cement without sand and gravel premixed in the bag. Quick set is also known as Calcium Aluminate. Or you could use some firebrick mortar but it usually contains some sand so you may need up to 25%.
What you'll have to do to get the right mix is to experiment. I'd start with 50/50 mix and see if it will stand up to the heat of your system. Make sure to pack the mix as well as you can into the mold. Any loosely packed areas will fail quickly.
Now to take you in another direction: many folks, myself included, have found that ceramic fiber blanket with or without rigidizer can make a very cheap and effective riser. Just get a 1" thick blanket and roll it up to line the inside of a black or galvanized stove pipe of the right size.
If you want to make a perlite clay riser here's how.
Your clay must be clean (no rocks) perlite is available in generous bags of 4' for around $20
Normally a #50 sack of dry fireclay is used. (Your clay can be dried and pulverized.)
Dry clay is sprinkled over perlite, add water. The mix is more perlite than clay. You want to keep the insulating property's of the perlite with the sturdiness of the clay to hold it together.
A softball size piece should hold together on its own but still bust apart if enough pressure is applied.
A round concrete form (sonatube) works well as a sacrificial inner liner , a 16 gallon barrel works well as an outer form.
As Duane mentioned, 1" thick ceramic blanket (called a 5 minute riser) inserted in stove pipe makes an indestructible riser.
They and ceramic fiber boards are the newest inovation in rocket construction.
I suggest using your local clay mixed with lots of sand as filler in your mass.
I'm including a few early photo's of my "Walker " style perlite riser.
They work well, but ... My new stove has a 5 minute riser.