Cristobal

I have finished the third firing. On the second one I have put 36 lbs on bottom and medium shelves. For bottom shelf 06 and 05 cones are fully collapsed. I'm waiting for the kiln to cool to see the results of the third firing which included some bricks with both stretcher and header (vertical) glazed. Tomorrow I will do the fourth one.

I have finished firing before midnight on Sunday. Usually I open the kiln on the morning of the next next day, so after 34 hours. In this case I had to wait longer, because even on Tuesday evening the pyrometer was still showing 55 C and I usually open when it's showing the room temperature (which is outside temperature in my case), probably because of large thermal mass of the glazed bricks. Only today morning, so after 58 hours it finally showed 27 C. The bricks on top shelf did not get (as usually) enough heat, but they will not be really visible in the project I'm making them for and I just glazed them translucent. Middle and bottom shelf got gorgeous deep cobalt blue. The bottom shelf got the 06 cone collapsed, so probably it reached 05 to 04. The bricks were still warmer than all other fired objects that I took out from the cooled down kiln in previous firings. Shelves were fine, but will inspect them in the daylight. It means that one 5/8 high alumina shelf with 4 posts could hold 29 lbs of material at cone 05. Next firing will have a little bit heavier load, because I will add shorter 5 lbs brick between middle posts. I still have to do 10 more firings. With the longer kiln cooling time I will be able to do it every third day, instead of every second one. I will post some pictures later.

I realized that my shelves are not cordierite, but regular high alumina. I resquared brick posts and made them more uniform in height. The old ones that I was using in the torch zone, under the first shelf, got shrunk by 2-3 mm from repeated firings and I cut new ones. I measured distances to the walls and used clay wads covered in kiln wash on two middle posts only. One middle post was too high (or shelf sagging a little in that spot) and I ground it with a diamond file to prevent any rocking. I loaded only 4 bricks per shelf, because the distance between middle posts was only 176 mm and my short bricks are 183. I have 168 mm bricks also and they would fit, but did not want to spend more time on painting them with the enamel. So I fit 4 * 2 * 3 = 24 bricks and am firing now. The pyrometer is showing 520 C.

Thank you Bill. I just checked and I can move the end posts 1' away from the edge and still fit 4 full and one short brick on the shelf. Mark, Bricks will be glazed on the stretcher side (the 2.5x9" side) and some both on stretcher and header side.

Mark, I will be glazing heavy firebricks that I purchased, not making them from scratch. If I did, I would use no shelves. Bill, I have found hot load strength for BNZ bricks (byr PTI Thermal). It says that at 2000 F at 10 psi load for 1.5 h it's 0.1%. 10 psi would be equivalent to 0.7 kg/cm2 so still two times more that my calculated load of 0.32 kg/cm2. I would like to do the firing as soon as possible, but other tasks are drawing me away from it.

I think it is Skutt 1227, 3 phase. I removed all electrical parts and converted to gas.

Bill, For posts I'm actually using pieces of firebrick that I cut to size. Piercing the bottom. Hmm.. I put entire kiln on 0.25" plate and the plate rests on the kiln legs. Cold crushing strength of IFB-2600 brick is on average 2.0 MPa, which is roughly 20 kg/cm2. My posts are 6.5x6.5 cm so the area affected is about 42 cm2 and for 4 posts it's 168 cm2. The weight of 14 bricks + 3 half shelves would be transferred to this area. The weight would be around 45 kg + 9 kg shelves = 54 kg. 54 kg distributed over 168 cm2 gives 0.32 kg/cm2, so 62 times less than the cold crushing strength.

Should I wait for them to dry? Could I use ceramic fiber instead?

Neil, I was actually worried about rocking possibility. What are "clay wads"?

Rae, I have 310 bricks to glaze. I can put 5 per shelf, it's 10 per bottom level, 10 per medium and I can only fit 8 on top level, due to flue shape (4 posts and 8x8" shelf on top). This way I will be able to fire 28 at once, so I will need at least 11 firings. Each firing takes me at least 3 days. I also work, take care of the homestead and help to finish the house. If I fit less then it will take proportionally longer. I will give it a try and if it fails or not, I will report here for reference. The middle shelf hardly gets cone 06 and the top one even less.

I will be glazing bricks. One weighs around 7 lbs. The half shelves are a little bit smaller now - I needed to trim the ends to provide better exhaust flow for better temperature uniformity in the kiln. Now they are 25x12", so it should help a little. I decided to add a fourth post - in the center of the shelf's semicirlce. This way I can fit 4 full bricks (2 on each side of the center) and one cut brick (between middle posts) on a shelf. In theory I could buy thicker shelves or even buy some more expensive ones, but I would need to do more test firings to get results I got after 7 tests and since my workshop is completely outside I can not waste another season, before the weather becomes wet.

I called them - they did not know, the guess was 15 lbs. I called my local ceramic store where I purchased the shelves and they said no more than 50 lbs I called another store in Bay Area and they said 25 lbs.

I'm trying to find cordierite shelf specification data to find out the maximum allowed weight that the shelf can withstand at a given temperature and 3 supporting columns. My half shelves are 26x13" and 5/8' thick, Firing up to cone 05 and planning to put 45 lbs on each shelf.

Min, The picture of the bricks in the link posted by Peter shows bricks with holes, so definitely machine made. The commercial clay pugmill probably has vacuum pump to remove any bubbles. Machine made, extruded firebricks glaze normally. I would like to mention zellige tiles - also made from terracota and their terracota looks very similar to my bricks with the same color variation and hardness when I cut them. It's possible that their clay was created by the same geological processes, from the same deposits in the similar climatic zone as Andalucia across the sea. They glaze beautifully - the artisans pound leather hard clay tile flattening it to the desired thickness and then they SMOOTH it with a tool, to close all macro pores.

Rae, I always applied three brush coats of Spectrum - the were absorbed the same way in my kiln - only once on top shelf at cone 05 they could be considered acceptable. The custom glaze I'm using is cone 08 borate-based and firing beautifully on firebricks on all shelves. I applied it by brushing and that was the first glaze that made me happy (after I tried three other ones, - frit-based). When I dipped them they also turned out beautiful between 05 and 08. Glaze density was around 1.7. I love this glaze, because it seems compatible with my rough conditions and the glaze works for me and not the opposite. For all things that I do (especially for gardening and farming), I'm always trying to find the limit "how much to baby things", but at the same time the act of babying things creates true masters... Babs, I'm considering to leave the Spanish bricks as they are, just seal/wax them. I just wanted to do something different with them, but maybe it was too pretentious anyway.