I wondered if you might know where the tonal range has gone in these natural raw clay slips?
I don't know anything, really, about natural clays from specific localities. It's a fairly safe bet that the colours are down to iron - they look that way
So why would they disappear when taken to 1230 with a transparent glaze over it.
Iron compounds are weak colourants, so paler colours often disappear, particularly at higher temperature and in the presence of a glaze. Also, mineral and glass compounds contain iron in two valence states, usually mixed. The different states find an equilibrium which changes according to temperature and chemical context. The different valence states are associated with different colours. Higher temperatures are associated with weaker cooler colours, lower temperatures with stronger warmer colours. Unglazed areas will sometimes partially revert during cooling but this becomes less likely with higher firings, largely due to gradually glass formation within the slip/body (vitrification). Glass protects the colourants from atmospheric oxygen, so glazed areas tend to preserve the iron in more or less the valence state it was in at top temperature
So at top temperature the weaker iron colour faded ans was 'fixed' by the glaze?
Yes. It's a partial reduction effect brought on purely by raising the temperature. There's a lot more to iron in ceramics - it has a strong attraction for calcium and for titanium, both of which are often present in glazes. High alkali content has an oxidising effect, etc., etc., etc. These associations all affect colour.