Callie Beller Diesel

Couple of areas where clay can be sensitive to fast heating, also slow at start, or even candling if you not sure of moisture content  and clay thickness. Some folk hold somewhere around 700-800 °C to drive off xhemically bound water but if you havent had to up until now, I wouldn't start.
Let's know how you go,centainly save on energy and time.

I believe that recommendation is to prevent the material from getting into the element grooves. I don't think that switching to another material is necessarily any better in that regard. If you're going to do it, use the material that works the best which would be silica sand, and just be sure not to get any into your elements, and vacuum them out after firing.
There is very little shrinkage in a bisque firing, so it's more about even heating/cooling and/or expansion/contraction during the heating/cooling.

Slow. Both for firing up and cooling down.
These pictures are from Digitalfire of lowfire ware, pots have cracked from uneven cooling, edges will have cooled faster than the middle area of the pieces. The way to avoid this is to have the cooling down go as evenly as possible. Slow the cooling down between 1150F - 950F, I go at 100F/hour through this zone with suspect work.

 There is negligible shrinkage, ie less than 1% between bone dry greenware and bisque firing to ^06- ^04, would be interesting to see data that shows refiring to a lower temp causes more shrinkage.
edit: when having your piece in the kiln put it between shelves, not at the top or bottom of the kiln, this helps the heat even out also. I put rectangular or triangular kiln posts around the outside edge of the shelf also. (round posts can roll)
 

Hi and welcome!
Just to clarify, do you want it to look different in the bucket, or in the end result?
If you want it to look different in the bucket, food colouring works a treat, and burns out.
If you want the end result to look different, a small quantity of a light blue stain would pop glaze colours better than a brown or grey one would.

Hi and welcome! I wish it was under better circumstances.
The pictures are worth a thousand words, and thank you so much for including those!
The fact that the piece is broken so cleanly, and in 2 near-perfect vertical lines all the way through means that this wasn’t specifically your clay, or anything you did during building the piece. It’s a nice dense clay  that probably stuck to the kiln shelf due to the mass and size of the piece, and cracked during cooling. For the next pieces, I’d fire them on some sand/alumina so that the piece has the equivalent of little refractory ball bearings to shift around on. You could also use a waster slab that will shrink at the same rate as the piece, but take the brunt of the force and absorb the crack instead.
If the clay survived the bisque just fine, another possibility is to not fire the piece to full clay maturity. Porosity in the end piece is less of a concern for you than it would be for someone throwing functional ware.

Interesting point. Our raku glazes do best at 1850 or maybe even a bit hotter than that. I typically judge by the look and not so much by the pyrometer, so maybe I can judge the color for obvara...

Hi and welcome! I wish it was under better circumstances.
The pictures are worth a thousand words, and thank you so much for including those!
The fact that the piece is broken so cleanly, and in 2 near-perfect vertical lines all the way through means that this wasn’t specifically your clay, or anything you did during building the piece. It’s a nice dense clay  that probably stuck to the kiln shelf due to the mass and size of the piece, and cracked during cooling. For the next pieces, I’d fire them on some sand/alumina so that the piece has the equivalent of little refractory ball bearings to shift around on. You could also use a waster slab that will shrink at the same rate as the piece, but take the brunt of the force and absorb the crack instead.
If the clay survived the bisque just fine, another possibility is to not fire the piece to full clay maturity. Porosity in the end piece is less of a concern for you than it would be for someone throwing functional ware.

Hi and welcome! I wish it was under better circumstances.
The pictures are worth a thousand words, and thank you so much for including those!
The fact that the piece is broken so cleanly, and in 2 near-perfect vertical lines all the way through means that this wasn’t specifically your clay, or anything you did during building the piece. It’s a nice dense clay  that probably stuck to the kiln shelf due to the mass and size of the piece, and cracked during cooling. For the next pieces, I’d fire them on some sand/alumina so that the piece has the equivalent of little refractory ball bearings to shift around on. You could also use a waster slab that will shrink at the same rate as the piece, but take the brunt of the force and absorb the crack instead.
If the clay survived the bisque just fine, another possibility is to not fire the piece to full clay maturity. Porosity in the end piece is less of a concern for you than it would be for someone throwing functional ware.

So I’m going to preface this by saying I’ve never fired Obvara, and I’ve only ever fired raku by eyeball, never with cones or thermocouple. (Please do this with proper eye protection). So I couldn’t tell you what temperature exactly I was using, but the glaze recipes used were mostly gerstley borate at the time, and that melts between 1550 and 1600 F. 
So my question is, can you fire the raku pieces to a lower temp to match the Obvara recommendations, or are you using glazes that really don’t mature until that hotter temp?
If you do need the hotter temp for raku, it’s possible to roughly judge the temp of a piece by the colour of the glow coming off it.  The  chart linked below has a nice colour gradient illustration, and you can do a bit of a comparison from there. There’s a paywall, but you can use one of your 3 free articles/month to view it. 
https://ceramicartsnetwork.org/ceramic-recipes/recipe/Kiln-Firing-Chart-142658

I’ve seen this form a bunch, and yeah, it’s tricky to get right. One solution for the warping I saw another potter online do was to do all the minimal trimming and cleanup as you describe, but they then added a slip trailed circle of clay as a foot rim. It was just enough to keep the full surface of the plate’s bottom from being in direct contact with the kiln shelf so you don’t get the warping, but still keeps the same aesthetic qualities of this style.
Results may vary with different clay bodies, but it’s something that’s worth a shot.

I first got hooked on clay because we did raku at my high school. The only time we had anything explode was the one time we tried firing a piece that wasn’t bisqued first. 

I’ve seen this form a bunch, and yeah, it’s tricky to get right. One solution for the warping I saw another potter online do was to do all the minimal trimming and cleanup as you describe, but they then added a slip trailed circle of clay as a foot rim. It was just enough to keep the full surface of the plate’s bottom from being in direct contact with the kiln shelf so you don’t get the warping, but still keeps the same aesthetic qualities of this style.
Results may vary with different clay bodies, but it’s something that’s worth a shot.

Hi and welcome! I wish it was under better circumstances.
The pictures are worth a thousand words, and thank you so much for including those!
The fact that the piece is broken so cleanly, and in 2 near-perfect vertical lines all the way through means that this wasn’t specifically your clay, or anything you did during building the piece. It’s a nice dense clay  that probably stuck to the kiln shelf due to the mass and size of the piece, and cracked during cooling. For the next pieces, I’d fire them on some sand/alumina so that the piece has the equivalent of little refractory ball bearings to shift around on. You could also use a waster slab that will shrink at the same rate as the piece, but take the brunt of the force and absorb the crack instead.
If the clay survived the bisque just fine, another possibility is to not fire the piece to full clay maturity. Porosity in the end piece is less of a concern for you than it would be for someone throwing functional ware.

I’ve seen this form a bunch, and yeah, it’s tricky to get right. One solution for the warping I saw another potter online do was to do all the minimal trimming and cleanup as you describe, but they then added a slip trailed circle of clay as a foot rim. It was just enough to keep the full surface of the plate’s bottom from being in direct contact with the kiln shelf so you don’t get the warping, but still keeps the same aesthetic qualities of this style.
Results may vary with different clay bodies, but it’s something that’s worth a shot.

I'll fire some chips in the bisque kiln this weekend.  If it is tannins they should burn out.  This is also city water, not well water, so I expect only low levels of tannins.

Bendy foam hair roller from the dollar store, I use them for supporting the rims of slab pots etc and also around the handles of the glaze tongs. I slit them up the side then tape them on, saves rubbing the skin sore between my thumb and index finger on a long glaze day.
 

While I add dry ingredients directly to my old batch at my personal studio all the time with no ill effects, if you’re in a teaching studio, you might want to mix the new batch separate and test it to verify everything went as expected before adding it to the old batch. And depending on how well the studio members/students are mixing the batches, you might want to let it run out as far as possible before adding new. If incomplete mixing is a habit, it can mess with the glaze.

While I add dry ingredients directly to my old batch at my personal studio all the time with no ill effects, if you’re in a teaching studio, you might want to mix the new batch separate and test it to verify everything went as expected before adding it to the old batch. And depending on how well the studio members/students are mixing the batches, you might want to let it run out as far as possible before adding new. If incomplete mixing is a habit, it can mess with the glaze.

This one from my students. If you manage to wear the skin off your pinky, vet wrap is a good way to protect it. 

This one from my students. If you manage to wear the skin off your pinky, vet wrap is a good way to protect it. 

This one from my students. If you manage to wear the skin off your pinky, vet wrap is a good way to protect it. 

This one from my students. If you manage to wear the skin off your pinky, vet wrap is a good way to protect it. 

Yes. 
Most glazes are just fine with running through an 80 mesh screen once, some run it through twice. I hold a bit of the water back to rinse the sieve out at the end of sieving to try and get as much out of it as possible. 

cool.  I wanted so much to know if it lived up to the hype.  have been building a fish all day, and because i needed to add feet (I always put feet on my fish), i have to let the body set up a while and add the feet, and then the fins--just did that, so we will see what it looks like in the morning.  here is the photo so far  so far just making what I can use up without letting it get stinky.

If you can’t get proper pottery plaster for a reclaim slab, you can lay an old sheet over wire racking and lay thick slurry out on that to dry.