Sir Richard
The KLM 747 at ANC report is
NTSB ANC90FA020. I can lend a bit of insight - the volcanic ash ingestion eroded the compressor, and upon passing through the burner, melted into glass droplets that tended to "plate" the nozzle guide vanes of the HP turbine. The combination of these two effects robbed the stall (surge) margin of the HP compressor system, and compressor stalls occurred. Some engines flamed out.
The shock cooling from the flameout caused crazing/cracking of the glass coating on the NGV's, and some of the glass flaked off and exited the tailpipe.
This had the effect of restoring
some of the stall margin, and it was possible to restart all engines after that.
This effect has been observed on other engines, notably in test bench ops subjected to dust- or sandstorms. I have little doubt this was the scenario in the BA 747 case.