Lots of questions. I don't think I have much by way of answers, but..... I do use lava rocks (aka scoria) and have a variety of different sizes.

Sometimes I use very small ones in the plant mix itself. These would be probably 5 mm and under. I use scoria mostly for top dressing and I do so because it is lighter in weight than washed pebbles or pea gravel, and also because I like the look better.

I put a lot of work into preparation of it though. I bought a couple of large bags and carefully unwrapped them to see the contents. Then I put the largest stones back into the bags and ran over the bags with my car, in the driveway. That did a pretty good job of making large lava rocks into small and medium-size ones. I sorted according to what I DETERMINED was size -- small, medium and large. Then I sifted everything and washed everything --and baked nothing.

For me, that's as far as it goes. What works, works. What doesn't gets repotted and changed. I only use red scoria so I have no idea about the gray. But -- good luck with however you decide to pursue this. Hope you come back and tell us about the results you got.

Hahah...you ran over the bag with your car! That's awesome.

Rinsing - yes, to get rid of dust...it may be accomplished by sifting. I sift anything chunky (incl. perlite). Sometimes I rinse, more often just sift. There is always dust in there.

Baking - IMO, not needed.

Otherwise, just what robin said...anything over 7-8mm is getting too big for the mix, but good for topdressing. Do not know anything about the grey...

1. Size

What is optimal particle size of lava rock for a small coffe mug sized plants. A uniform particle size of .100 (1/10 inch) ensures the medium cannot hold excess/ perched water in the spaces between soil particles. "Optimal" would be no particles smaller than .100 (2.5mm) and all particles as close to .100" as possible. It's not a perfect world, so you'll need to decide how close to perfect is practical for you, but if you're to err, it should be on the side of particles being slightly >.100 as opposed to smaller.

I actually sifted my red lava rock by few sizes because I could not find any smaller than 1-5mm grade:

below 4,5mm (arround 1/6 inch)

between 4,5 to 7mm (slighly less than 2/7 inch)

and above 7mm (which im sure is too big, it really looks like that big chunky dog food some people have for their pets) Unless the pot/plant combo is very large, the upper particle size should be limited to particles <.200 (5mm) for best results.

Should I use the smallest one or can I combine it with the medium one, since there is very little of those finer particles and I really dont feel like crushing that lava. That would be up to you. I would try to find another material closer in size to the ideal ..... perhaps some mixture of crushed granite, pumice, Turface, pine/fir bark ...... Using materials with different water holding characteristics provides you with the ability to 'adjust' water retention by varying the ratio of ingredients.

2. Washing

I've heard it contains a lot of usefull minerals for plants, but at the same time if I wash all this dust it is kinda like washing all those minerals away, is that dust really that bad for drainage and if anything blocking those holes in lava rocks that it needs to be washed? Yes. Even a 1/16" layer of sludge covering the bottom of the pot can provide an environment which favors any of several fungal pathogens gathered under the umbrella of damping-off diseases. Rocks are not candy and minerals already have really hard time disolving from them, that dust would be literally the only source of those minerals if any. Even if the dust was as fine as talc, the particles would still be hundreds of times too large to be beneficial to the plant, and very little mineralization will occur in containers where the environment is more hostile than welcoming to soil life, especially if the medium is comprised of all or almost all inorganic materials. That isn't a problem, at all, as that's what fertilizers are for. The grower would do well to stay focused on how the grow medium is structured and not its ability to provide nutrients to the plant. Growing in containers is much closer to hydroponics than growing in the earth/garden.

Also I assume the dust, especially the finest one might increase CEC (cation exchange capacity) since it is like clay and clay has high CEC, I plan to combine it with coco coir, which has good CEC but not the greatest tho and since I'm watering with hydroponic nutriens, I want to avoid fertiliser burn, which high CEC helps avoid I urge you to avoid sacrificing drainage on the altar of nutrient retention, and thinking mineral dust in your medium will have potential to be anything but limiting. You can use a calcined clay or DE product which gives you added CEC w/o having to worry about the sludge.

Are you sure you want to use coir as the only organic fraction of the medium? Why go through all the trouble of worrying about particle size and building an 'ideal' medium when you intend to undo all your well intentioned efforts by filling up the spaces between particles with coir?

Coir has many issues that would need workarounds. It's high pH precludes use of dolomite as a liming agent, it is very high in K, very low in Ca, often very high in soluble salts, can be so high in Mn that Fe uptake becomes an antagonistic issue. Fresh coir also contains allelochemicals (toxic to plants susceptible to the chemicals) that affect a large enough % of species that it shouldn't be ignored. Most growers think they'll simply replace peat with coir and the plant won't notice. It will.

Also the dust kinda stops those sharp edges from being sharp What sharp edges do you refer to? You could grow a perfectly healthy plant in a tub of broken glass if you had willingness to keep up with the watering it would require (think hydroponics).

3. Porosity

While the red lava rock has a lot of nice holes, the grey one literally has almost none, I was kinda disappointed when I opened the bag, because it really looks like any random gravel from an unpaved side of a road, also after washing small amount to see how it looks like, unlike red lava rock and pumice it looked like the water stayed on its surface... Did I just buy a bag of kinda expensive grit or there is more to it? There is more to it.

4. Desinfection

Can I bake my lava rock in the oven, and if so on what temperature... I'm quite sure its ok but it will never hurt to ask... No need.

Al

Thanks @tapla (mid-Michigan, USDA z5b-6a), your answer was extremely helpful, and exactly the kind of help I was looking for!

I'm very grateful for that!

There are few things I want to say, as I didn't specify exact soil mix I planned to use and details about it...

You asked me why would I ruin my perfect soil particle mix by using small organics like coir...

Well I did not know that much about particle size importance when I was buying stuff for it, also lot of people made very simillar mixes

(I based my mix on youtuber "Cactimania" one that used 1/4 organics like loam compost or coco coir with worm droppings and 3/4 lava pumice and grit, since he lives in UK and I in Poland and our weather is slightly more dry so I wanted to maybe make it slightly less fast drying so it would hold water a bit longer - hence the 1/3 organic...)

I also heard a lot of good stuff about coco coir an how much plants love it, I've heard it also holds air pretty good and most importantly isnt that annoying to water as peat, as I absolutely hate how it shrinks, becomes hydrophobic and sticks to roots like it had some glue in it

about the dust on lava I was just really curious especially that many sources stated it has a lot of those minerals but after your recomendations and the ph I discovered it had I defintely gonna wash this stuff off

I was planning to make a mix 1/3 organic or 1/4 organic and it follows something like that:

1 Part Coco Coir + 0.5 part Worm Droppings

1 Part Red Lava Rock (I ll try to do something about the size)

1 Part (or 1,5 Part) Pumice (1-5mm)

0.5 Part that weird grey lava rock (1-5mm)

About the Coco Coir:

I bought Plagron Coco brix which are supposed to be buffered with calcium, as well have adjusted ph, trichodermia added, in general they are supposed to be highly processed and I've read good reviews about it...

I wanted to use it since I've heard that it retains water in a good way but also its quite loose so it still holds air and it is less annoying than peat to water

About the worm dropings:

I dont know what to say exactly, they were supposed to be "soil conditioner" and "add organics" into the mix

They are also from Plagron since I was buying from one hydroponics shop, they look to have a lot o peat in them and dont disolve a lot of stuff in water, since it only looks murky from the dust but doesn't change color that much... On the website it doesn't provide much info either and says that they dont provide much nutriens as a fertiliser... Next time If I will be using them I will buy local ones, that appear to be better quality and dont have much peat and rather actual droppings high in organics made from horse manure processed by worms... In general I planned to use them more as weak organic fertiliser for the coir so it isn't totally empty...

Also I decided to measure ph of the lava and rest of the stuff and oh boy, I'm making sure to wash all this dust off!

While the ph of gray lava dust appeared to be slighly basic but close to 7 (maybe 7.5?) the ph of red lava dust was definitely above 8, probably solid 9 since it was all nice blue on the indicator

the ph of Coco coir was something between 5-6

and worm dropings were something arround 6,5 maybe 7? - they were hard to measure

the organic stuff had ph as advertised while the lava was supposed to be neutral - DEFINITELY wasn't (ofc I used good quality deionised water, but had only liquid ph indicator and those ph strips that are just a pain and a mess, I kinda need to buy digital ph meter)

here are the results if you are curious and wanna see the mess I made trying to measure ph of these things:

https://imgur.com/a/pD88EDc

So what I'm gonna do now? Im not sure exactly...

I've read your responce as well as link included and there was A LOT of info to take in, so I kinda need to read it few times again

You recommend me using other things than coir, like fine pine/fir bark that I would need to buy (but also find in right size, which can be sometimes a pain like with that lava rocks)...

Im definitelly still gonna use that lava and pumice but defintelly gonna wash it

I was trying to create really good mix, maybe not the perfect one, also I wanted something that actually will hold good amount of water (but not in a soaking wet way but rather lightly moist but for longer) and still provide air between the water holding medium...

Im not that concerned about the medium decomposition as I plan to repot them more oftem like every other year

my cacti even in store bought soil they had when bought grow really fast (altrough they are in very small pots and when I water them I touch the soil with paper towel to drain the excess - since its completly detached from the pot and in one piece) that is the reason why I really need to repot them as I did not expect them to outgrow the original pots and soil that fast...

Soooooo as I understand

I could use Coir + wick combination to drain excess water and maybe even use it to water plants through it (I actually thought about this so maybe I could water my plants with less effort)

Or pine bark instead and water normally

I kinda do not know what to do since I totally wasn't concerned about coir and rather just on lava, now Im like back to the hanging point but instead of thinking of lava I think of coir...

My only concern is that the pine bark chips will dry too fast tho

Again I have to read your link once more and think about this, also Its quite late and I need to go to sleep...

Thanks again for the answer and info!