The black soot is from carbon. Either the flame is too large for the combustion chamber, the droplet size is too big, there is not enough combustion air or there isn’t turbulent enough mixing of the combustion air with the fuel (rich and lean parts of the flame). By the way, red/brown soot or scale is usually from too much excess air when burning a fuel that contains sulphur. What type of burner is in your range cooker, what is it’s firing rate and how big is the combustion chamber? There are addatives in cooking oil that helps it last longer. That may be part of the white ash.
The black soot doesn’t bother me anymore, because I now use a siphon burner. The siphon produces the white soot/ash. The firebrick target is like covered with chalk powder. I spoke an old man who used to burn WMO and he had also a white combustion chamber. In his opinion it’s white because of the high combustion temperature. I think it is a combination of high temperature and heavy oil containing more carbon compared to hydrogen. I am burning hydraulic fluid at the moment. Will soon use WVO when a friend has taken over a fast food shop. Our range cooker is a Heritage. Beautiful oldfashioned English design. You can see a picture on www.heritagecookers.co.uk . We have the standard cooking-only model. It is on 24/7. They are also available with an extra build-in burner for domestic hot water and central heating. So one machine doing it all. The burner is a small Swedish Bentone, type ST 40 HSW ET. Nozzle .35 GPH 80*. Flame length about 4”. Combustion chamber about 7”x7”x7” (estimated). I spoke the technical director of Heritage recently and they are experimenting with SVO. It seems to burn well with only a nozzle heater. I can believe that, because the burner is in the warm cooker housing, so oil and air are preheated by the cooker itself. We are burning HHO, but in England mostly kerosene # 1 is used.
I’m burning 25%hydrogenated oil and 75% HHO in a stock setup and I get the white flaky ash too so it must be part of the animal in it..lol Note; With that note the flame does shoot out of the fire chamber a bit because it makes it longer but great btu’s and starts & burns great with a .85 or a .75 – 70 deg. Nozzle. One thing I do is only mix 5 gal at a time to prevent long term setting which last me min 1 ½ days and up to 4 days if I’m not here much I turn the stat down to 45 and even off all day. The oil fired hot water tank heater with a .65-60 nozzle ( Carlin burner) don’t like that mix at all if the house is below 70′ for some time. It won’t start unless I heat the line some but with no wife to complain and only heat it when I want it, it’s no big deal asp. Because its not in the basement.