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Pieter Mostert

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Posts posted by Pieter Mostert

  1. 1 hour ago, MFP said:

    Hmm...plastic vitrox? Is there another name for it? I think not as I do not recall  that material.  I cannot remember if I ever used it for anything. You guys on here are so sophisticated compared to the time I worked.  We were "make do with what you have" potters....and the simpler the better.  Glaze theory was only done at universities.  And everything was a big secret!

    PV clay? https://digitalfire.com/4sight/material/pv_clay_1152.html

  2. Was the unknown ingredient Plastic Vitrox clay? Sounds like it might be this recipe: https://glazy.org/recipes/2875

    Be aware that the analysis in glazy uses the theoretical composition of Colemanite (https://glazy.org/materials/15119) whereas what's actually sold may have a different composition, for example https://glazy.org/materials/37220 or https://glazy.org/materials/20787

    If the 35 year old piece is functional and has been in use for most of its life, I wouldn't worry about any theoretical durability concerns.

  3. 22 hours ago, Bill Kielb said:

    @Pieter Mostert

    The way I keep it straight in my head:

    If the glaze crazes it is because the glaze shrinks faster (more) than the  Clay. The glaze is in tension but the tension exceeds the strength of the glaze so it cracks. If the glaze shrinks very very slightly faster (more) than the clay, the clay is in compression, glaze in slight tension but the tension does not exceed the strength of the glaze.  It now becomes the tension member in our prestressed concrete or a rubber band around our ware.

    when glazes go in compression the opposite is happening and will shiver off the pot when compressed enough. A good way I envision this is a buckle in the pavement. On a hot summer day the surface expands so much it puts itself into compression and since it is constrained eventually exceeds the modulus of rupture of the material and it erupts or buckles upward where not constrained.

    to answer your last question, actual tests verify that claybodies in slight compression are up to 20% stronger.

    Are you sure the tests didn't involve glazes in slight compression? I would think that, given the relative thicknesses of glaze and claybody, the compressive force exerted by a glaze on a claybody would be minimal.

    I did a bit of digging, and came across this Alfred Masters thesis by Jennifer Benson. After skimming through this, it's still 100% clear to me why an increase in the compression of a glaze should increase the strength of the ware (up to a point), but I think it may be due to the fact that this reduces the likelihood of cracks originating in the glaze.

  4. 14 hours ago, Bill Kielb said:


    An additional helpful view of this from the compression standpoint: if we can keep concrete in compression, suddenly it can flex like a diving board (extreme example). Prestressed concrete is created on this basis and allows thinner sections to develop more ultimate bending strength. .

    There  are many examples of prestressed flexible concrete on you tube today which still amaze me that the thing stays together because the cables or tension members are stretched so tight that the concrete cannot go into tension.

    A nice glaze squeezing your pot ever so slightly really increases the strength In much the same way.

    A non-crazed glaze is (probably) not under tension, and may be under slight compression. But if the glaze is under compression because the clay-body contracted more than the glaze, wouldn't the clay-body be under tension? I've also heard the claim that glazes under slight compression strengthen ware, and while I don't doubt that it's true, I can't figure out why this is.

  5. If you want to start with a solid cube with 40 cm sides, you'd need over 100 kg of clay (assuming the density of clay you're using is similar to that given here). I suggest starting with a shape that's more like a box with thick walls. With 30 kg of clay, you can make a cubical box with 40 cm sides and walls 8 - 9 cm thick. This will also make it easier to join the separate pieces of clay together.

    Having groggy clay helps, but I think you'll need to be very careful about how you dry the pot to stop it cracking.

  6. 7 hours ago, Babs said:

    In good company then!:-))

    Maybe you need a Chinese glaze for it to work.....

    Getting above temp of hot water....

    In an oven I put the ink on when warm/ fresh out of the kiln....not a cool unpacked here

    I wonder if a higher temperature simply makes the ink less viscous, allowing it to penetrate the cracks better, even if they're narrower. This link doesn't specify what type of 'ink black' they're measuring, but it shows a dramatic decrease in viscosity between 20 C and 50 C: https://wiki.anton-paar.com/en/ink/

  7. 10 hours ago, liambesaw said:

    Could be because the clay body expands more than the glaze when heated?  If you were heating only the glaze it would be like what you were saying? Seems like there's a lot of room for difference experiences based on the body and glaze involved

    If the clay body expanded more than the glaze when heated, it would shrink more than the glaze when cooled, so the glaze would be under compression, and wouldn't craze.

  8. 6 hours ago, Babs said:

    Expands the cracks a bit so ink can get into the cracks a bit easier but care as too hot and it' s hard to get the ink off the glaze.

    I've found that heating does the opposite. In one crazed vase I have, the cracks disappear when you soak it in hot water, but then reappear once it's cooled. This makes sense to me, since there's no tension in the glaze at the temperature that it solidifies, but then the tension increases as you lower the temperature, due to the difference in thermal expansion between the glaze and body. So heating the pot should reduce the tension, and therefore shrink the cracks.

    But you aren't the only one who suggests heating a pot expands the cracks; Nigel Wood talks about this in his book on Chinese Glazes.

  9. Curt, I gave a description of the tile in my last post: Silica increases from left to right, and Whiting increases from top to bottom. So it isn't a Currie grid, but the bottom left corner still has the most flux. This is a useful test when your starting glaze doesn't have much clay, but has relatively high alumina.

    Mary, I fired the tile flat. This is not an example that shows increased fluidity, but I can dig up some if you're interested. I should add that the results of firing flat vs vertically can be fairly different. There's some discussion in the Currie thread about how to get the most info from flat tiles, including some indication of fluidity.

    By the way, the reason Insight gives different UMF numbers than what you calculated is that it doesn't include iron as a flux.

    Glazenerd, I haven't heard of iron being involved in shivering. I thought lithium was the main suspect.

  10. 16 hours ago, glazenerd said:

     Calcium stiffens a glaze, in this case the opposite is required.

    That's not always the case. I've done several line blends (at cone 4) where I just added increasing amounts of Whiting to a glaze, resulting in an increase in fluidity, up to a point.  If you keep adding Whiting, sooner or later your glaze will become underfired, since Whiting on its own has a pretty high melting point.

    What sticks out for me from the Insight analysis is the high UMF value of alumina. Have a look at the glazes plotted here (you can refine them to show only cone 6 if you like. I assume that's what you're firing to). There aren't many with Al2O3 as high as 0.99, and most of those that do, have high KNaO.  Adding Wollastonite will bring down the Al2O3 UMF value, in addition to increasing to Al2O3 : SiO2 ratio and getting the flux ratio closer to 0.3 : 0.7 (Insight rounds off the numbers). But it might be more informative to do a biaxial test where you add both Whiting and Silica. I've attached a test I did where I increased Silica going left to right, and increased Whiting going top to bottom.

    I'm not claiming you'll get a stable glaze this way, since I think Matt recommends some boron for cone 6 glazes. But I also think the tests he did on stability didn't involve iron, so I'd be reluctant to  extrapolate from them. I should point out that I haven't taken his course, so I could be completely wrong about this.

    BOB biaxial.jpg

  11. This is great!  I made a similar form using google sheets, but it's almost impossible to use on my phone, so hopefully something like this will work (I haven't tried this on my phone yet). Do you mind if I fork your repository to adapt it to my version?

    You can find my spreadsheet here: https://docs.google.com/spreadsheets/d/11WFZMq3A6ZtTCneMBRO_5DCOBuNJ3SICMvPkA2YtgB4/edit#gid=0

    Actually there are two versions. The second one uses a two-step procedure for when it's impractical to weigh the glaze.

  12. Joel is right.

    You've probably seen this in recipes that came from Glazy, which used to describe certain ingredients as theoretical, although this seems to have changed recently. The problem is that some recipes just call for something like 'potash feldspar' without specifying which brand. In order to work out the Seger formula for that recipe, you need to know (or make assumptions about) the composition of oxides in each ingredient. 'Theoretical' potash feldspar just meant that the composition of pure, i.e. "theoretical", potash feldspar was used to crunch the numbers.

    In practice, think of it as a warning that your brand of potash feldspar (or whatever) may give different results than the brand used by the author of the recipe. (Also, don't trust the given Seger formula 100%)

  13. I have this model, but it came with the older DTC 800C controller, which lasted 4 firings before it went completely haywire. I got an electrical engineer friend to replace it with a PID controller, which hasn't given any serious trouble. According to the previous owner, the original controller had been working fine, but the kiln had been in storage for 18 months before I bought it. If possible, I'd test the controller by programming a short ramp with a hold, and checking that the kiln follows this. If you do this, make sure you read the manual beforehand.

    I need an extended firing to reach cone 4, but that's probably because the floor is cracked and the bricks are out of alignment on a couple of corners, so there's a gap that opens when the kiln heats up. Some of the element grooves are quite close to the top of the bricks they're in, and for a number of these bricks, the part above the groove has broken off. This was because the previous owner transported it with shelves (wrapped in newsprint) inside, and lifted it out of the car at an angle :(. I should have know better too, and told them to only put balled-up newsprint inside. Anyway, I don't think this will be a problem if you don't do anything stupid, but just be aware that that's something to look out for. If the kiln and elements are in good condition, I don't think you'll have trouble reaching cone 6.

  14. On 08/05/2015 at 5:45 AM, curt said:

    Finally, in the near future (five year?), the kinds of household batteries which Tesla, Enphase, SMA and other are bringing to the mass market will make it completely feasible to run a kiln off grid. 

    Turns out it took less than 3 years.  And you can use some heavy 3-phase machinery at the same time.


  15. 4 hours ago, Joseph F said:


    I think this is right now. I plugged in your formula Pieter and it checks out with the math on my currie page for additions. Glaze Measuring ( 4th page ) 



    Based on the formula you use for F4 in the Glaze Measuring spreadsheet, cell B4 should be the weight of glaze per cup.

  16. On 03/03/2018 at 10:44 PM, kraythe said:

     Then measure off smaller batches of 140g of glaze which should be 100g of base

    If your s.g. is 1.4, then assuming the relative density of the materials in your glaze is 2.6 (see the pdf Min linked to), you'll only have 65g of dry materials in 140g of glaze.

    You can work this out as follows: Let D be the weight of the dry materials, and W be the weight of water (in grams). Then since the relative density of the dry materials is 2.6, the volume of the dry materials in the glaze is D/2.6, and since the relative density of water is 1, the volume of water is W/1 = W (in mililitres).  Therefore the weight of the glaze is D + W, and the volume of the glaze is D/2.6 + W, so its specific gravity is

    sg = (D + W) / (D/2.6 + W)

    With a bit of algebra, you can express D in terms of sg and W, or W in terms of sg and D. However, in practice, you usually only know the specific gravity and the total weight, T, of the glaze. In this case,  since T = D + W, you have W = T - D, which when you plug into the formula above, gives

    sg = T / (D/2.6 + T - D)

          = T / (T + (1/2.6 - 1)*D)

    Now you can solve for D in terms of T and sg:

    D = T * (sg - 1)  / sg / (1 - 1/2.6 )

       = (13 / 8) * T * (sg - 1) / sg

    Plug in T = 140 and sg = 1.4, and you'll get D = 65.

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