Callie Beller Diesel

Well it's looking like the second quarter in terms of Pottery sales has been slow-same as the first quarter. This is showing in my retail/wholesale outlets . The economy in our small backwater community is always lagging behind the national scene . Its been a bust and boom economy localy for all my time living here. This area was once king to Redwood timberr sales (when I moved here over 53 years ago). Then that went bust and Weed growing took over until thats gone bust in past years. Now its in between  the next thing. The toursit industry has been steady thru it all and thats a season flow for te shops and galleries . Now that I no longer travel with pots out of the area so I'm  now subject once again to this cycle except to one very large wholesale gallery far away from here wich I'm working on a huge twice a year order right now. In terms of my slowing down work wise this actually has worked well for me..I need to stock up for the summer trade season soon aas well. Not sure about te rest of the country but here pottery sales are slower than past years at this time locally.

If the food colouring is just to tint the raw slip then I would suggest using paste food colouring rather than the liquid, it goes much further.

A waster slab should do the trick, or put some silica sand on the shelf. Make sure you put kiln wash or alumina wax on the waster slab so the tile doesn't stick to it. If you need a cooling ramp it would be during quartz inversion, so from about 1100 to 900, but it's rare that you would need that.

I used the cones and they bent as per the temperatures. But I am going to run few tests and also maybe aad a new program because I am also having issues with pinholes lately. Thank you!

Claybodies should be vitrified enough to not weep / leak even with no glaze on them when used for functional work and fired to mid or highfire.
Is this the clay you used? https://tuckers-pottery-supplies-inc.shoplightspeed.com/mid-smooth-stone-speck-clay-cone-6-stoneware-ecom.html
If that is the clay then in theory the clay should be vitrified enough to not leak with a posted absorption figure of 1.0 % but published figures can vary by +/- 1%. I would suggest running your own absorption tests. This is a good idea to to when using a new clay or every once in a while on an existing clay as materials can change.
How to on testing for absorption here if you need it, about 2/3 the way down the page, I would suggest doing the weight calculation test with several samples placed in different areas of the kiln. 

If she isn’t absolutely 110% sure it’s a cone 10 clay then I would pass on this. 
This makes sense to fire the clay to maturity / cone 10 if she doesn’t plan on glazing it and it is indeed a cone 10 clay. She might want to leave it as bare clay or be planning on doing a cold finish on it. Firing to maturity would be a clearer way to describe what she wants rather than calling it a bisque firing. A mature body is far stronger than a bisque fired body.

An relevance? https://www.lickinflames.com/obvara_june_2021.html
Generally speaking, the hotter the pot going into the mixture, the darker it will be. Nearly glossy black pots are from the hotter side of the firing range and tan pots are usually from the cooler side of the range. Really hot pots held in the mixture longer will yield fewer patterns. Cooler pots held in the mixture longer will possibly not carbonize as they cool below the threshold where carbonizing can take place.

What type of kiln is it? If it has a door, just pull the obvara and then close it up and finish the raku pieces. Otherwise I think you'd probably be fine just firing up to raku temps and pulling the raku pieces first while the obvara cools a bit. It's not an exact temperature anyway, since the pieces cools some between the kiln and the bucket, and the last piece is always a lot cooler than the first.

Another tip would be to leave a “button” in the center bottom of the plate. You can find some images of Steven hills plates/platters to get the idea. 

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.