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Technical Tips Blog
This electric kiln thermocouple needs replacement
The old one inside is in bad condition (a new one is sitting on top ready to install). In 2022 these cost about $35 CDN. The temperature-measuring part of a thermocouple is the join of two dissimilar metal wires, these are 8 gauge. The junction produces a temperature-dependent voltage that a pyrometer can convert to a reading. Thermocouples can degrade into pretty poor condition yet still work, notice the one in this kiln is separating in two. Thermocouples generally need replacement more often than elements. Notice the elements in this kiln are laying down, they also need replacement.
Friday 25th November 2022
Using clay with children
It is often said: "No good deed goes unpunished"! This can happen when doing a clay session with children. So there is a need to enter, with "both eyes open" to avoid a negative experience. If you can succeed they will get a unique tactile experience in a social setting. And they will experience the anticipation of waiting to see what the kiln will do with their creation. If the kiln gifts them a brightly colored durable piece, especially one that they can drink or eat out of, the experience and the object can stay with them for a lifetime. If you are an instructor inheriting a kiln and clay program for kids it can seem overwhelming, especially if your technical knowledge is limited. But there are some simple things you can do to assess the condition of the kiln in your facility and whether it is practical to attempt some clay sessions with the children in your trust. Click the link below to learn more.
Context: Working with children
Thursday 24th November 2022
This amazing difference 45 micron silica can make
The only difference between these two cone 6 glazes is the silica. Both are the G2926B recipe, both were thickly applied and fired in the same kiln. The left one employs the 90 micron (200 mesh) grade silica and the right one uses 45 micron (325) mesh. These test tiles are about 6 months old. There was no crazing out of the kiln. The porcelain recipe is 25% silica, 25% nepheline and the remainder kaolin and bentonite. It appears the finer particle size silica is dissolving in the melt much better, this narrows the difference between calculated and actual behavior, especially relating to coefficient of thermal expansion.
Monday 21st November 2022
Do these kiln elements need replacing?
This 12 inch test kiln has done 910 firings. The element loops are laying down and nearly touching each other. If they are not changed soon the coils will touch the kiln will have hot spots. And the coils are expanding and getting tighter in the grooves, the longer we wait the more the grooves will be damaged when removing them. Although elements seem expensive, when costed on a per/firing basis they can be surprisingly inexpensive. Most hobby kilns service two elements with each relay and relays generally need to be replaced more often than elements. Consider, for example, replacing the elements on a Skutt 818. Being a smaller kiln it is well-powered in relation to size and elements can last up to 1000 firings (assuming 50:50 bisque and cone 6 firings). It has 4 elements and 2 relays (relays cost $65/ea, elements $95 each). The labor to replace is ~4 hours or $250 - total cost is about $750 (that is ~75¢ per firing or 32¢ per ft³). How about a larger kiln? An 8 ft³ Model 1222 has 5 elements and 3 relays and replacement is ~$1100. But its elements are only likely to last 200 firings. That yields a per firing cost of ~$5 and per ft³ of 65¢. But there is a much greater cost to consider: Old elements increase power consumption. An 818 uses 6.4 kwH and a 1222 uses 11.5 kwH - at our electricity cost of 14¢/kwH a firing costs ~$7 for the small kiln and ~$13 for the large one. But that is the cost when elements are new. When they need changing those numbers can more than double! An additional cost of old elements is ware consistency, the kiln cannot execute the firing schedule in the time programmed and this will likely affect the appearance of bodies and glazes.
Thursday 17th November 2022
A load of 800 bags of silica arriving at Plainsman Clays
Each of these bags is batch-numbered and time-stamped, this is an impressive product-tracking effort by US Silica. It appears the time changes once per minute and from our calculation, their plant was producing about 600 bags/hour. They stack silica 60 bags on a pallet, 3000 lbs! The only material we receive that is stacked more densely than that is plaster, 80 bags per pallet at 4000 lbs. We stack our clay at 40 boxes/pallet giving 1760 lbs.
Tuesday 15th November 2022
Cones bending badly, cones bending goodly
Ware is not turning out as expected and a potter needs to verify the temperature in the kiln. The standard cones on the upper right are misleading. The cone 7 is telling one story but the cone 6 and 5 another. On the lower right is a better way: Self supporting cones. They are always at the right angle and this set of three is bending as expected. To be a full cone 6 the middle one needs to bend just a little more until the tip is even with the top of the base (maybe 2 or 3 degrees). On the top set, the cone 6 is clearly totally flattened and the 5 is a pool of glass, this firing went way beyond cone 6.
Tuesday 15th November 2022
Control gel by adding Veegum as the glaze is mixing
For a 340g powder batch (to get a pint) of this G1214Z1 brushing glaze (it has 5% titanium added) my target is 5g CMC gum and 5g Veegum. CMC controls drying speed and Veegum the amount of gelling. I already mixed the CMC with the powder, shook the whole batch in a plastic bag and aded it to the 440g water. Now I am adding the VeeGum slowly, while blender mixing on high speed, this enables tuning the degree of gel. Because this recipe has little clay, it took all 5g of Veegum. Drying between coats was faster than normal (so it could have taken a little more CMC gum), but I was able to apply three coats to a bisqued piece in only a couple of minutes. Good recording keeping is essential when doing this because observations of drying speed and settling vs over-gelling are important to determining the right gum amounts on subsequent batches. On the second day I had to update my notes because it had gelled more overnight. So next time I will try 4 grams of Veegum.
Wednesday 2nd November 2022
Is the N505 cone 6 matte glaze recipe what you think it is?
This recipe is from page 2 of the booklet: "15 Tried & True Cone 6 Glaze Recipes". Click the following code, G3955, to see more information on how we compare it with G2934 and another matte, G1214Z1. This flow test and these test tiles were in the same kiln, fired at cone 6 using our PLC6DS schedule. Obviously, the defining characteristic of N505 is its extreme melt fluidity, clearly it is not a native cone 6 glaze (it's a lower temperature one being used above its range). Still, the surface on the N505 tile is arguably more interesting, that's why it's popular with potters. But is it functional? Some felt pen marking helps reveal one big difference: The micro surface of the G2934 is much smoother. From the chemistry shown on Insight-Live side-by-side screenshot very low Al2O3 and SiO2 are evident. This also reveals it should fire glossy, so it is a "crowbar matte", forced to be such by 6% addition of magnesium carbonate. I was understandably suspicious that this glaze would have more issues than it actually does. Although the surface is rough and it does mark and stain, it can be cleaned with effort. The low SiO2 suggests it would cutlery mark but it does seem quite hard. However on the matter of leaching the jury is still out (a stain needs to be added for testing in an acid). Crazing is another possible issue. Our G3924 recipe, although more boring, excels on all four of these tests.
Sunday 30th October 2022
The glaze cost on this mug is three times the cost of the clay!
This jar of glaze will do seven of these mugs! Four coats are required because it is watery. What about the time? To glaze a thin-walled piece like this could take an entire morning of applying coats and waiting for them to dry, more time than actually making the piece itself. There are two obvious choices for a more economical and faster method: Make your own higher specific gravity brushing glaze and do it in two or three coats, heating the piece to about 200F between each. Or heat the piece once to 250F and quickly immerse it in a dipping glaze and be done in 10 seconds! Our G3879 recipe is a good starting point for both options.
Friday 28th October 2022
G2917 floating blue as a dipping and brushing glaze at cone 6
The dipping glaze version of G2917 was used on the left, we make that by the bucket and it drains and dries in seconds after dipping bisque ware. We also make jars of the brushing glaze version in our studio (it is not sold). That was used on the outside of the mug on the right . Of course, it is much slower to apply but there are some advantages. First, it was easy to control the thickness to maximize the variegation this glaze gives with thin and thick, revealing the throwing lines better. And where thicker application was needed (e.g. at the rim) it was easier to achieve that. And near the foot ring it has been applied in a thinner layer. And the brush strokes do impart a more handmade look that is also nice.
Context: Three rutile blue glazes at cone 6
Thursday 27th October 2022
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