https://www.generalpumice.com/

Have you tried General Pumice? I’ve purchased pumice from them before, but only their ready made products. Nothing so specific as what you’re looking for or if they can do a custom request.

@SoCal Stewart (San Diego, Ca Zone 10A/10B) Yes I tried General Pumice and they had no interest in doing anything custom for me

Why not just buy a bunch of pumice and make your own? GPP probably doesn't find it economically feasible to do this for anything less than a ton of sand.

@cactusmcharris, interior BC Z4/5 That is what I have been doing, but I need this stuff 15 gallons at a time. I spend $20 to buy the pumice and much more in time to make it.

There must be at least 10 people on Houzz who already buy Dry Stall and filter it? I am offering to pay them for time they are going to spend anyway.

I was not sure what the product was so I looked it up, it looks like a gray to beige porous material. Reason I mention that is, I thought it might be worthwhile to look at sandblasting media, since some of them are made from pumice or other volcanic materials. I don't think they use the same stuff though. A quick search for the black stuff I have for sandblasting indicates it's an aluminum oxide, and they also use crushed glass and other sands. Porous pumice like you're using would not be structurally tough enough to use in blasting. Also the particle sizes are much smaller than the size range you are looking for. 102 mm is about 10 to 20 grit, and blasting media are more like 60-80 grit.

Makes me wonder though, where else you might find such a material in an unusual place.

Is there a way to further crush the bigger particles? Like, find a rock analysis lab that has a crusher? A glass recycler? Someone who would run a few bags through their crusher for you, in exchange for a box of donuts. :-]

One place, http://global-abrasive.com/index.php/blasing-media has 3 sizes of pumice and it might be what you are looking for, but I don't understand their size ranges (1/2N, 01/2D, F).

OK Now I have the sizing key.

https://www.kramerindustriesonline.com/product/pumice-grit/

So it looks like #6 pumice is pretty close - 10 to 30 mesh (0.6 - 2mm). The picture sure looks like just a finer size of what Dry Stall is, and they describe it as a 'light, soft' abrasive. Might be just what you're looking for. Assuming it doesn't cost an arm and a leg.

@toxcrusadr So my problem is not figuring out a procedure to get from a starting pumice material to where I want to end up. I know how to get from A to B to C. My problem is that getting from A to B to C is costing me 10 times as much money as the cost of the product being fed into step A. :) It will be cheaper for me to buy the finished material than to keep wasting my time making it.

@toxcrusadr So Kramer's product looks very close, but I would still need to filter it to keep the smallest particles around 1.2mm. I would end up wasting half the material. In addition, the price of 55 cents per pound is actually expensive. I can get a similar product from a large pumice mine in Idaho, but again I would gladly pay someone to do the filtering for me.

Why not just buy it from the people who mine it?

http://soilrox.com/sizes-applications-buy.html

Back when I was building my rhododendron beds using Permatill and Turface, I considered buying a pallet of pumice from Hess and having it trucked across the country.

@UpperBayGardener (zone 7) I have pricing from Hess Pumice, the mine that makes that product. The main problem is you end up having to order a full pallet from them to make the economics work, and that is almost a $1000 order including shipping cost.

Westes, thanks for reminding me of the correct spelling of pallet! Good luck with your project!