To summarize the ash removal steps:
Cool the shell after the burnout process
Clean the remaining ash out of the shell using either compress air or by rinsing with water
Once the combustion is complete, there will be a small amount of ash in the shell.
Ash removal is a significant disruption to the process, typically requiring the shell to be cooled before ash removal. If the ash removal step could be skipped, then the vents could simply be patched (although carefully since the shell is hot) and the casting poured. Not only would it save time and effort, it would eliminate the concern of cristobalite conversion and weak shells.
If the casting is poured without removing the ash, it typically will float to the top of the melt and result in some pitting on the surface of the casting, as shown in the photo. For some applications, the resulting minor surface imperfections are acceptable. Even if the resulting surface is not acceptable, it may be possible to repair the surface with minor welding.
For the majority of applications, however, it will be necessary to remove the ash to minimize imperfections in the casting. There are differing opinions on how best to remove the ash.
One foundry uses a method that it is best to remove the ash by blowing out the shell with compressed air while it is still hot. They mentioned that the ash is fine and feathery while hot, but as the shell cools, it is more likely to stick to the walls of the shell and be much harder to remove. To be sure they have removed as much ash as possible, however, they still allow the shell to cool and rinse it afterwards.
Most of the foundries either blow the shell out with compressed air, or rinse the shell with water. With either method, the shell is first cooled to a temperature at which it can be easily handled, usually near room temperature.
If compressed air is to be used, it is best to remove the ash before the vents are capped. That way, air blown into the sprue passage will blow ash out the vents and vice versa. The vents allow for flow through the cavity. If the vents are capped first, it will be much more difficult to remove the ash.
If water is to be used, it is best to cap the vents first so that water doesn’t leak out of the shell in several places. Once the vents are capped, the water can be poured into the shell, swished around to pick up any ash that is there, and simply poured out. Foundries often pour the water out through a sieve fitted with a filter cloth to see how much ash was in the shell.
To be thorough, one might use both methods;
Blow out the shell with compressed air
Cap the vents
Rinse with water
Some foundries use boiling water to rinse the shell or even citrus solutions.