davidh4976
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Posts posted by davidh4976
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Is the glaze coming off during the firing and landing on the kiln shelf? If that is the case, it can be due to the way the wet glaze dries and cracks and falls off.
If the glaze is not falling off onto the shelves, but is leaving bare spots, that can be a different cause.
I can't tell which is happening from the description or photos.
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3 hours ago, PeterH said:
Just for the record Ceramic Arts Daily publish "15 tried and true glaze recipes" for (at least) raku, cone 6 & cone 10.
Ah! I missed that there are three different versions of their "15" recipes! All of the raku recipes have less than 20% clay, so extra clay is probably not the problem.
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I think the "Ceramic Arts Daily publication of 15 tried and true glaze recipes" are all cone 6 recipes and would not be well suited for Raku. I don't know about the Gary Ferguson recipes. Can you share your recipes here and we can take a look at them? Since the flaking is happening with both dipped and brushed, it makes me suspect the amount of clay in the recipe. With too much clay, the glaze can shrink during drying, crack, and flake off before firing.
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22 hours ago, neilestrick said:
I think you'll just have to try it. My gut says 3 kiln is too many, though, and you'll get freezing.
My gut also says that freezing is possible. Since no one is confidently saying that it's no prob running three raku kilns off one 100# tank, I think I will abandon the idea of using a single 100# tank. I would hate to invest in the 100# tank, regulator, and plumbing and then find out it freezes up!
Our alternative is to continue using the 20# tanks; one for each kiln, but we will put the tanks in tubs of water with an aquarium water heater in each. I think this will work because we have only had the freeze ups in colder weather.
I do aquarium heaters with my personal gas kiln and I have never had freeze up. My configuration on my personal gas kiln is a TDI downdraft converted Skutt 1027 using two 20# tanks ganged together. Both tanks sit in a tub with an aquarium heater. I start at 0.5psi and run up to 3psi on a single Ward MR750 to get to cone 10 in 5 hours using less than 4 gallons of propane. No freeze ups.
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14 hours ago, Bill Kielb said:
Do you know for sure how many btu your present burners are rated? If not you can sneak up on it with how many firings in a 20# new cylinder. Conservatively you will likely get down to 1/3 - 1/4 of full charge before it becomes difficult to use, especially at 40 degrees. There are about 4.6 gallons (440 kbtu) in a 20# tank so basically 5 times more in your 100# tank.
They are Ward MR750 burners. We run them starting at 1 psi, going up to 5 or 6 psi. Ward says the MR750 is rated at 54,365 BTU at 1psi and up to 153,768 BTU at 8psi. That matches our fuel usage that we measure by weighing the tanks and confirming with the amount of fuel put in each tank when it is refilled.
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6 hours ago, neilestrick said:
I think you'll just have to try it. My gut says 3 kiln is too many, though, and you'll get freezing.
How is your kiln constructed? 40 minutes to an hour seems a little long for a typical raku kiln.
We have a few different kinds. One is a 55 gallon drum lined with ceramic fiber; about 4 cu Ft. We are moving to only having this style. We are experimenting with ramp-up times will probably up faster in the future.
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Because we are doing raku, we run the MR750 burners at higher pressure. Starting at 1 psi and going up to 5 or 6 psi. (From Ward regarding raku: "With 1-8 PSI, the MR750 Single System is rated at 54,365 to 153,768 BTU's".) The firings last from about an hour to about 40 minutes each depending on if it is the first firing or subsequent firings. I'm guessing we are averaging about 100,000 BTU per kiln per firing which tracks about right with our fuel usage. If we do three firings on each of three kilns, that should be about 10 gallons so a 100 pound tank has enough fuel.
My bigger concern is tank freezing. My understanding is that using a 2 stage regulator system will help prevent freezing. I also think I understand that the tendency for a tank to freeze is related to the surface area of the liquid in the tank. A 100 pound tank has about 50% more liquid surface area than a 25 pound tank. What I don't know is how all of this translates to real world experience! (We've had a 25 pound tank with single stage regulator freeze up in 40F weather, but the tank on the next kiln over did not freeze up.)
I'm seeing 1st stage regulators that handle from 1,500,000 to 2,250,000 BTU/hour depending on the model all at 10psi. They are all in the same price range, so I assume that the higher the BTU/hour, the better. So, my conceptual configuration is shaping up as: I'd use a 100 pound tank with a 1st stage regulator reducing the pressure to 10psi, then a manifold with individual valves for each kiln, with hoses leading to adjustable regulators (0-to-30psi) near each kiln, followed by the MR750 burners. I have to figure out my hose lengths and therefore sizes.
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If I wanted to simultaneously run three raku kilns off of one 100 pound tank, how would the plumbing work? A regulator at the tank to drop the pressure to something like 10psi and then an additional regulator for each kiln? Would I use a fixed 10 psi stage 1 regulator at the tank and a 'red-top' adjustable regulator for each individual kiln? Would I be concerned about tank/regulator freezing when feeding three raku kilns simultaneously from one 100 pound tank (assuming up to 6 psi for each kiln with a single MR750 burner for each kiln)? We are just brainstorming about whether to switch from three individual 20 pound tanks to a single 100 pound tank.
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11 hours ago, PeterH said:
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.Thanks, that's very useful info!
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You might have a relay that is stuck in the closed/on position.
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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...
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At our next raku firing, I'm thinking of trying obvara. Obvara is fired to 1650F while raku fires to 1850F. I'm wondering if I can successfully fire obvara and raku pieces together to 1850F, but allow the obvara to cool before immersion in the obvara fluid. Thoughts? Any idea of what happens if the obvara pieces are hotter than 1650 when dunked? I thought about using my infrared thermometer gun to read the temperature of the obvara pieces, but it only goes up to 842F, so I can only guess at the temperature of the Obvara pieces.
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I like the idea of soft bricks as thermal shock shields. We've been using cookies, but they are a pain to remove/replace between firings. The soft bricks would be a lot easier to handle.
We are possibly going to have a firing with everything being glazed immediately before the firing. So, based on everyone's feedback, I think I'll try candling the first load by running it at about 200F for 45 minutes before starting the regular cycles of firing and pre-heating the pieces on top of the kiln.
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Anyone have experience with glazing pieces immediately before putting them into a Raku firing? Do the pieces mostly survive OK?
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Neil mentioned that "A lot of people also put a shelf at the top, like 3 inches below the flue opening."
Rae said "A Soft brick damper is easier to slide, but can drop debris onto pots on the top shelf (speaking from experience). Keep top of kiln as clean as possible. "
Both are good points. I just thought I'd add a fancy drawing to illustrate what Neal described. The top shelf is reported to keep some of the heat in the kiln better but still allow the right air flow. It should also address Rae's point about debris. I don't know from personal experience since my gas kiln is a downdraft, but it makes sense to me. In your test firings, how much space is between the kiln shelves and the sides of the kiln?
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There should be adjustable shutters on the bottom of the burners to adjust the primary air. You may need to have those more open to get better combustion at higher gas flows. Once you get those openings set correctly for the top of your firing range, you can leave them alone thereafter and kick them down. Get them a bit loose while cool before the firing and then play with them at the top of your firing to see if you can improve things. They will be hot, so a long screwdriver might work on the adjustments.
Another thought is that the kiln exit hole in the kiln lid may be a little too small for higher altitude altitude. You may need to enlarge it. Not a huge amount. You can always reduce its size with a damper during firing.
For a damper, I find that on a kiln like this, a soft fire brick is easier to handle and maneuver than a piece of a shelf. Either can work.
Also note that a Type K thermocouple can be inaccurate above cone 6. I don't know if it's universal, but my use of Type K thermocouples above cone 6 is that they read low. You have to use cones and look at them through a peephole to really judge. Or use an expensive Type S thermocouple.
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The bricks are quite fragile, so in addition to the advice already given, do not attempt to lay the kiln sections on their side. Even when hand carrying the sections, do not hold them any way except upright and flat. When hand carrying, keeping the sections on a piece of plywood will help. All of this will keep the kiln walls from flexing. Flexing of the kiln walls can do a number on the firebricks.
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Is this a glaze that has previously worked well for you and suddenly gone bad, or a glaze that has had this problem ever since you started using it?
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32 minutes ago, Min said:
@davidh4976, good job on subbing the Custer out. I like how much silica and alumina are in the formula, will make for a good durable glaze. I could see a bit of iron in the recipe if you ever wanted to tone down the blue a little.
Thanks! Everyone in the pottery LOVES the color, so I am not tweaking the color at all! :-)
I general, I am finding that Al2O3 of about 0.4 and SiO2 of about 3.5 (UMF) is working well for our "workhorse" cone 6 glazes.
I may play with increasing the B2O3 flux a bit to see if I can get the glaze to smooth out on the Chocolate clay body without detrimentally impacting the color and coverage on the other clay bodies.
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20 hours ago, Hulk said:
I wonder if my glaze fitting struggles were related to 200 mesh silica (which is what I have)?
We had a situation about a year ago where we started to get crazing on glazes that had never crazed before. It took a lot of trial and error testing with known good batches of materials to determine the problem. It turns out that we had received a batch of 200 mesh silica by mistake instead of 325 mesh. Because there was no clear "mesh" marking on the bag, no one realized that we had received 200. The glaze mixers had unknowingly started using 200 instead of 325 in the glaze mixes. Switching back to 325 solved the problem. Of note, the crazing happened only on cone 6. We did not get crazing on cone 10. My theory is that cone 6 was not hot enough to melt the 200 mesh completely, but cone 10 was able to completely melt all of the 200 mesh.
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After a bit of testing, I found a Cone 6 recipe that gives a good, deep blue color that stays blue even over edges/texture. I avoided Gerstley Borate, Neph Sy, and Talc. GB because of end-of-production. Neph Sy because it somewhat contributes to hard panning. Talc because of recent quality. This has been tested and found to be craze free (boiling water/ice water test) on the following clay bodies from our local supplier, New Mexico Clay: Marilyn's BOD, Ochre, E-Speckled, Cashmere, and Chocolate (these are all of the Cone 6 clay bodies that we now support in our pottery). On Chocolate, the glaze surface does not look as smooth as on the other four which look great. Here are the two recipe versions, one using Mahavir Feldspar and the other using G-200 Feldspar (because we have both in our supply!).
AMT MATERIALS
27.6 MAHAVIR POTASH FELDSPAR
26.5 SILICA 325
15.3 FERRO FRIT 3134
18.4 EPK
12.2 DOLOMITE
2.0 COBALT CARBONATE
102.0 TOTAL
AMT MATERIALS
29.3 G-200 FELDSPAR
26.2 SILICA 325
13.9 FERRO FRIT 3134
16.4 EPK
12.2 DOLOMITE
2.0 COBALT CARBONATE
100.0 TOTAL
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We have two glazes that settle very quickly (one recipe below) compared to the rest of our glazes. What ingredients cause a glaze to settle and/or hard pan faster than other ingredients?
From my collection of notes from various sources, I suspect the following, but would appreciate your feedback:
- Dissolved sodium, such as from Nepheline Syenite and some Frits (3110) can cause deflocculation. Deflocculated glaze will hard-pan.
- Other materials that can encourage hard-panning are: wood ash, soda feldspar, nepheline syenite, lithium carbonate, and many common frits such as 3110 (644?)
- Insufficient clay in a glaze (I add 2% Bentonite to recipes with none or very little clay, and 1% Bentonite for recipes with less than 10% clay. If the recipe has over 10% clay, I do not add Bentonite.)
I have been reformulating our glazes because of the shortage of Custer Feldspar and Gerstley Borate and have avoided the above items with good results on the glaze results and on keeping the glaze in suspension.
This is one of the two recipes that settles quickly compared to our other recipes. (The other glaze that settles has the same base, but slightly different colorants/oxides.) These two recipes are the only ones that use Frit 3124, so I am considering reformulating without Frit 3124. Does Frit 3124 contribute to settling/hard-panning?
silica 16.000 15.38% whiting 16.000 15.38% epk 10.000 9.62% ferro frit 3124 9.000 8.65% talc 9.000 8.65% G-200 EU Feldspar 40.000 38.46% red iron oxide 2.000 1.92% bentonite 2.000 1.92%
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Is it correct to say that if a glaze/clay combination does not craze at cone 5, then it should not craze at a higher cone firing?
If I normally fire to cone 6 and want to make sure that the new glaze/clay combo does not craze because of underfiring or a cool spot in the kiln, I think I can fire test pieces to cone 5 and then my normal cone 6 firings would be craze free. I use the alternating boiling/ice water baths for craze testing.
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Minimize bending the clay while rolling it out or when moving it from the roller to the drying boards. Dry between sheets of newspaper sandwiched between drywall boards or cement board untill completely dry. Change boards when they are damp by changing the top board, flipping the sandwich, and then changing the other board which is now on top. Allow to completely dry before firing. Fire on edge. Yes, fire on edge.
3 Raku kilns, one 100pound tank?
in Equipment Use and Repair
Posted
Everyone' comments have convinced me that a100 pound tank for all three is unlikely to work.
We are going to go with an individual 20 pound tank for each kiln with the tank placed in a tub of water warmed by an aquarium heater. In the past, we have run a good number of raku firings with 20 pound tanks and no heaters and we only had one freeze up on a day when the ambient temperature was in the low 40s.
I successfully use an aquarium heater on a small downdraft kiln that I fire on a pair of 20 gallon tanks for 5-to-6 hours to get to cone 10. The heater does a good job of keeping the water in the tub in the 70s.