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In my experience, the more technology is involved with something, the further removed you are from the realities of what you're working on. An example is wood working. A table saw, router, and screws, removes the carpenter from the nature and feel of the wood. One upon a time, all carpentry was done with wedges, froes, chisels, planes, etc. Ripping (sawing with the grain) was unheard of. You used a froe and riving brake for that. Items and structures this way are inherently stronger than and superior to those made with sawn timber. It was the law that parliamentary furniture in the UK had to be made with split timbers until it wasn't feasible to do so anymore. I feel the same about glaze software. Even the best of it tends to be buggy, fussy, and prone to a lot of transcription errors. I don't find it saves me too much time if I'm serious about a glaze, and I also just enjoy math and being hands on about things. For this reason, I thought I'd share how I like to do my glaze calculations. I don't tend to adhere to the usual limit formulae, because they make boring glazes, but in a lot of academic literature, glazes are either expressed as analysis (% by weight) or in unity formulas. That is, they're useful for replicating effects without being tied to a fixed material set. If you get really good, and have a little geological know how, you can actually take mineralogical analyses of chunks of random rock, and formulate glazes around them (or fit them to existing glaze recipes), figuring out how much feldspar, silica, lime, etc are in the rock. So this is how I do glaze calcs, with just a periodic table, some chem. knowledge, a pencil, paper, and an antique solar powered calculator. The info I've presented here comes from Digitalfire's discussion of unity formulae, Linda Arbuckle's discussion of the same, and Michael Cardew's Pioneer Pottery. All present basically the same stuff, but I find each presentation a little opaque in its own way. I'm going to keep things simple, and I'll start with a theoretically pure mineral to keep the math convenient and short. Let's say we have the following analysis for a feldspar: SiO2: 68.74% ; Al2O3 19.44% ; Na2O 11.82% . Theoretical soda feldspar. To be any use in calculation, we need to convert this to a unity formula. Hermann Seger was the first to express things this way, and it's based on the theoretical K-Spar formula 1 K2O. 1 Al2O3. 6 SiO2. The idea is that all the ROs and R2Os add up to 1 and the rest are expressed in terms of this as a ratio. But, this is expressed in terms of numbers of molecules, not molecular weight. So we need to figure that out. Period table time. I'll do soda and just give the rest. Sodium has an atomic weight of 22.9898 g/mol, oxygen 15.999. There are two atoms sodium, one of oxygen, so that's 2(22.9898) + 15.9999 = 61.9796. The molecular weight of "soda" is 61.9796 g/mol--we'll knock that back to 61.98 to keep the math nicer. I should add that a mole is a fixed number of atoms or molecules. Alumina's molecular weight is 101.96, and Silica's is 60.08. Knowing this, we can say the percentage is a portion of a 100 gram sample of the feldspar. So, 68.74/60.08 = 1.1441. There is 1.1441 mol of Silica in our feldspar. 11.82/61.98 = 0.1907. There is 0.1907 mol of Soda. 19.44/101.96 = 0.1907. There is 0.1907 mol of alumina. To get the unity, you would add up all the fluxes (RO's and R2O's) and divide each individual flux by the sum. There is only one flux in my example, but if there were two or more (as there almost always is), you would add up all the molar amounts and then divide each by the sum. Like, if there were 0.2 mol CaO and 0.1 mol Na20, you'd add those to get 0.3 and then divide both to get your unified fluxes (0.66666 Ca0, 0.3333 Na20). But, since we just have the soda, we just get 1. Then, we divide the silica and alumina by the 0.1907 (our sum of fluxes). We get 5.999 Silica, and 1 Alumina. The formula for theoretical soda spar: 1 Na2O. 1 Al2O3. 6 (ish) SiO2. Then, we want the formula weight. We multiply each by its molar mass, and add the products. 61.98 + 101.96 + 5.999 (60.08) = 524.35992. We'll call it 524.36. Note that Tony Hansen is a little off this, because, I think, of a transcription error. Ingredients that are pure flux, like limestone, are just a 1. The one exception to this unity is kaolin, which is expressed in terms of "ideal" kaolin, 1 Al2O3. 2 SiO2. The actual formula is something like 1 Al2O3. 1.996 SiO2 But now we have all the info we need to make a glaze. In this case, a 4-3-2-1 cone 8-10. What I call "toilet bowl clear." 40% spar, 30% quartz, 20% limestone, 10% kaolin. Make a chart, with the materials down one side, and the ROs, R2Os, RO2s, and R2O3s all expressed across the top, like below. Divide each percent by the formula weight, to get the equivalent weight. And then multiply the equivalent weight by each member of the unity formula. Plug the product into the relevant box. For the soda feldspar, this means 1 x 0.07628 for the Na2O box, 1 x 0.07628 for the Al2O3 box, and 5.999 x 0.07628 for the SiO2 box. When you have all the relevant boxes filled. You do the same unifying procedure as you did above. Add all the fluxes together to get a sum, then divide each flux by this sum. We get 0.27628. And 0.2/0.27628 = 0.7239 (CaO), then 0.07628/0.27628 = 0.2761 (Na2O). These total 1 for unity. Then 1.036/0.27628 = 3.75 (ish) for SiO2. And finally 0.4188 for Al2O3. So our unity formula is CaO: 0.7239; Na2O: 0.2761; SiO2: 3.75; Al2O3: 0.4188. Which ain't bad for a cone 10 glaze according to established limits. Our "toilet bowl clear" glaze should be a success--according to the math, at any rate. This is a grossly simplified example, but I encourage you to seek out this info on Tony Hansen's site, and anywhere you can so that you can use a pencil and paper to do this on your own, with your own numbers. Even if you do it only once. It will help your understanding of glazes immensely. And you can seriously use anything you have an analysis for to make a glaze. Get a rock analyzed and go from there. Math and Science are fun!
I have found several pages that all repeat the same: OK, fine by me Lets say we have a 250 l kiln (9 cf). We are going to need 16000 x 9= 144000 BTU We know that 1 Kg of propane packs about 13.97 kWh or 47668 BTU So, 144000/47668= 3.02 Kg (6.65 lbs) of propane per firing to cone 10? (I am using weight and not the volume because volume can be anything if exact pressure and temperature are unknown) Q 1: How close is this to actual gas usage? About the downdraft kilns and actual stacking area. Q 2: Is it safe to say that 250 l total inside volume will give us only about 155 l of stack space? I am asking, because we like to make some firing cost calculations before we start taking bids for building a 250 l kiln... or do we actually need a 350+ l kiln Thank you.