I have this happen sometimes with darker clay bodies. As was said above, it's usually scumming; but it can also be dust / detritus loose in the kiln from pieces from a white body, or even kiln wash if you're unlucky.
I'd say about 80% of the time, I can fix it with some vigorous sanding in the bisque stage. Obviously take proper precautions to not breath the dust from sanding.
If this doesn't work, then your best bet is to cover it up. It can absolutely show in the final firing.
I hate to state the obvious, but I don't think many people on here are medical doctors. Your best bet would be to see a dermatologist or your family physician.
It could be something as simple as irritated skin from grog / clay / constant moisture / etc... but it could also be an allergic reaction, or if your clay or glazes have specific materials that you're sensitive to. Who knows?
I have seen large slipcasting molds like this, but remember that much plaster is HEAVY... There's usually rigs for holding / pouring out slip, large catch buckets, etc. It's definitely a process.
I've also seen people do large pieces like this with slipcast sections. Split the vases into two halves and join them (slip & score), blend the line, and you won't be able to tell much of a difference.
Either way, it's a good bit of work. But definitely do-able.
But they really aren't that expensive (Blick has them listed as a set of 6 colors for around $45). Probably not the cheapest retailer, but they also have frequent 30-50% sales that I take advantage of.
It'll be cheapear / easier than tweaking formulas for colorant percentages, firing at different temperatures for the right hardness, etc.
Another option could be your pyrometer being miscalibrated. You're firing to 998o but if the pyrometer has aged or something has affected the voltage across it, then that could be inaccurate. Worth checking on too.
It could also be a clay body difference. Contact the manufacturer and see if anything has been reformulated recently.
I don't believe anyone has addressed the issue that immediately springs to my mind.....
How are you loading these kilns, with regards to shelf placement?
You said that the bottom of the kiln is a full 1.5-2 cones lower... This has absolutely been my experience if you place shelves close together at the bottom of the kiln.
This may not be the case for you, but I just hadn't seen anyone propose that as a cause yet.
Since (on an elementary level), heat rises, the bottom of the kiln will tend to be cooler. You can mitigate this by creating larger "chambers" at the bottom of the kiln throughout which heat can circulate.
But if you have a few 4" layers of plates/flat ornaments at the bottom, with tall objects on top, the top of the kiln will absolutely fire 2 cones hotter.
I started to throw a pot and it collapsed on me, I really like the way it looks this way and i have plans for it, the bottom still really thick, so I need to trim some out, my question is How do I trim this, I dont have a Chuck big enough for it, any ideas? Thanks, and may the clay Gods be with you.
I actually trim things upright pretty often... Especially when I don't have the right-sized chuck and don't want to use a ton of clay to make one.
You need a non-absorbant bat, and just a little bit of water on the bat (I usually just wipe it with a sopping sponge).
Level the bottom of the pot as much as you can, and then (with both hands) rub it in small circles on the bat+water. A lot of people use this technique to smooth out / level the bottom of pots.
If you keep at it, there will be a point where the clay absorbs most of the water and adheres to the bat. It will become VERY hard to move, but you can still do small adjustments to center it.
I trim it upright like this, and then do any finishing on the bottom by hand... Carving out any extra clay on the bottom by hand with foam and a banding wheel.