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Found 10 results

  1. @glazenerd I use a local South African white midrange stoneware clay which fires to cone 6. It is extremely short and I read that adding a 50/50 ball clay to bentonite mix will improve the plasticity. They recommend to add 50g to a kilogram of clay. Will this influence the cone my clay fires to? At first I thought it was short because it was freshly made, but I'm 6 months down the line and it's still short. It even cracks when I wedge and it's not dry. I think it might have neph sye in it because if I wedge normally it cracks like crazy, but when I stack and slam it becomes pudding like.
  2. Hi all, I'm based up in Finland and we have a lot of very clean local clay deposits. It's grey and fires to a dark red brown at the top end of it's range which I would guess is around 1100C looking at the glow. (the test we did was in a self built wood downdraft kiln without any cones or thermocouple. It started to boil and melt) There was a brick works on the site where I got the clay up until the '50's, so I know the clay is usable in some form or another. There is a LOT of clay around here, I guess as a result of glaciation? I dug a heap of clay about 15 years ago and it's been wrapped up outside getting deep frozen every year (-40C) I dried and washed a bucketful a few years ago which has been stored indoors since then. After hand wedging the clay it has a good smooth consistency but it feels quite strange. It almost like a gel?? It cracks and breaks as soon as you try to stretch it, but when you make a large ball of it, it starts to sag. If you manipulate it very slowly, say by squeezing, it deforms readily without cracking? It is not sticky at all, although in the bucket it looks like it should be as there is a fair bit of surface water on the clay. I don't have much experience with raw clay but I guess this needs an addition of a plastifier like Bentonite or Ball Clay or something? So a couple of questions... Should the clay be dried and powdered first? Or can I just work out the water content and wedge the appropriate amount of plastifier into it? ....and what percentage of plastifier would be a good place to start? Thanks in advance Rod
  3. I'm working with a clay that has a tendency to flop with little warning. From what little I know this can be a problem with plastic clay bodies holding more water than more forgiving, less plastic bodies. So with this in mind I'm looking at making adjustments but I'm getting conflicting information on what some accepted water content values are. Plainsman P700 has a % water of 23.5 - 24.5 Plainsman describes P700 as "our most vitreous cone 10 white body, it is the closest thing we have to a true translucent porcelain body. It is a mix of 50% Grolleg kaolin with feldspar and silica. We also add micro-fine bentonite to improve its plasticity." In the Febreuary Ceramics Monthly (thanks Tom) the water contents are significantly higher than the figures from Plainsman. I'm wondering are clays so different in the Unites States? I'm curious because the clay I'm working with holds aproximately 26% and is mixed up as a high fire, buff, stoneware. It works well for the most part. Fires to an 11 with no issues and kitchen tested absorption showed it was near zero at cone 10. But it doesn't stand up as well as the commercial Plainsman clays and I want to know why. thoughts much appreciated 20 epk 20 OM-4 ball clay 20 silica 20 custer potash feldspar 12 hawthorn Bond Clay 8 pv clay Percentage Analysis by weight 71.13 % SiO2 23.24 % Al2O3 3.06 % K2O 0.73 % Na2O 0.28 % CaO 0.32 % MgO 0.59 % Fe2O3 0.05 % P2O5 0.59 % TiO2
  4. Is there any scientific way of measure the plasticity and tell it in numbers ? I know the easy way by rolling out a coil and wrap around the fingers to check the plasticity in no time but I wanted to know is there any way that I can measure it and keep noted so that it can be a reference is future ...
  5. Introduction of SAS Formulation - hypothesis The industry standard has been formulation based on particle size distribution (PSD), which includes density packing. The Zameck article proposed formulation based on the PSD principle; which included emphasis on density packing. However, particle size is a measurement that only determines the plane (face) of a particle, but does not include the depth. A sugar cube represents a perfectly square particle: which has the same plane and depth equal. Clays however can have the same particle size on the face, but vary widely in depth. So a large grain of clay could be represented by a sugar cube shape: but in reality the depth can be anywhere from a thickness of hair up to a normal sugar cube shape. So using PSD as a determining factor for packing density is inaccurate; because it does not include the measurement of depth (platelet.) Specific Area Surface (SAS) is:Specific surface area (SAS) is a property of solids defined as the total surface area of a material per unit of mass. The SSA can be simply calculated from a particle size distribution, making some assumption about the particle shape. The SAS is far more accurate, in that it takes into account both the width (particle size) and the depth (platelet size) of clay. In common calculations: a potter will factor in a 200 mesh clay: but has no idea if that platelet size is 20-50-100-1200 mesh thick. So determining packing density solely on the basis of particle size is inaccurate, because platelet size is not factored. A 200 mesh clay could have a SAS as low as 18, and as high as 28: which this variance alone confirms the inaccuracy of using particle size alone. In addition to using the SAS in determining accurate grain size; the SAS will also give the potter insight into the plasticity of the clay of choice. The higher the SAS goes, the more plastic the clay will be. (Applicable to ball clays and bentones primarily). More importantly, the SAS becomes the basis of formulation because it factors total grain size. So from here, the SAS formulation needs to be applied.. Nerd
  6. Forum has been a little slow lately; so its a good time to ask this question: How do you define plasticity? Practical, technical, or common answers are fine by me. Curious to hear what plasticity means in the pottery world in general. Nerd
  7. I have been studying plasticity in stoneware bodies, as most know. I am finding some results that are making me question the accepted belief that plasticity equates to ease of throwing. Plasticity in general comes from the electrostatic charges on the clay particles; which changes as the body ages. I am looking for articles that specifically review the relation of sub micron ball clays, to the ease of throwing. I am trying to determine/figure out how mass plays a role in throwing. Ron Roy and I had this discussion at NCECA; what is the cut-off point for large and intermediate mesh sizes, before those additions create a denser mass: which makes the clay harder to push around on a wheel. It is very common for stoneware bodies to have 80% total clay content, there are some even higher than that. So I still find myself questioning if mass is playing the larger role in determining if a clay is easier to push around? Not sure if I am articulating my thoughts correctly, but hopefully I have made the question clear enough. As a comparison: everyone knows how easily porcelain moves around when thrown. The most common analogy is that it throws like cream cheese. That is because porcelain in general has 25% silica, and 25% feldspar; which has much less mass than fire clay. One of the major differences is mass: stoneware has more clay content; and much larger particle sizes. I have tested this theory by adding V-gum and macaloid to high percentage formulas of fire clay/intermediate clay. These additions are not the norm; solely done to test if plasticity is the determining factor in ease of throwing. Nerd
  8. Originally published in February 2019 issue of Ceramics Monthly, pages 66,67, & 68. http://www.ceramicsmonthly.org . Copyright, The American Ceramic Society. Reprinted with permission." techno File section: Clay Body Shopping Nerd
  9. Hello everyone! I'm getting better and better at throwing clay, and I have a question about what clay bodies everyone uses to throw. I have been using Standard 213 cone 6 porcelain, and I can't help but notice that the clay is stiff, and doesn't stretch very well. Looking at some videos online, I have seen how elastic some of the clay seems. The video in the link below is what inspired me to start throwing in the first place. Look at how great the plasticity of that clay is! I have attached a picture of the box I threw last night. I rolled a slab for the top and rolled a texture mat into it. It is the first one I have attempted. Trimmed and chattered this morning. I'm getting really excited about my gradually improving abilities, and I want to expand my scope of projects. The box in the picture was made with the 213 from standard clay. Do any of you have any suggestions for a more plastic high-fire clay body? I realize that shrinkage and warpage is an issue with a more plastic clay, but I would really like to know your thoughts on the clay bodies you like. Thanks very much! You all have been very helpful!
  10. First off, i would like to say hello to everybody. i'm new to the forum and haven't had the chance to fully explore the site. so consider this my formal greeting (with a dire plea for help). i'm a beginner when it comes to dealing with any kind of clay but i read a lot. i am currently trying to work with natural mined clay (dug out of the ground with my own hands). it's smelly but i can see it's potential. i have been able to refine the clay and remove all the things i don't want in there. my only real problem i'm having now is my level of plasticity. my clay won't pass the finger test (it cracks every time you bend it. unless you add way to much water). i'm hoping ya'll can help me. is there anything i can add to the clay to fix this. i know that i could age the clay but sadly i don't have 6 months+ to allow it to ferment and mature. i've read that i can add calcium lignosulphonate (apparently its a concrete additive that allows for less water and greater absorption. please correct me if i'm wrong) and it will greatly improve the plasticity, but i can't find it in quantities smaller than 1 metric ton. can i use ashes (i think i remember a teacher in school saying you could add willow or cat-o-nine tail fluff but once again i'm unsure and not willing to ruin my clay.)? somebody told me i could add sand but it seems to me that adding that would have the opposite effect. i feel as though i have exhausted google and youtube and i still don't have a solution to my problem and i don't know anybody with the experience to help me. i have 80+lbs of clay just waiting for the right answer so i can finally start creating things (and not having them want to crack apart while molding let alone when drying and firing). i want to make clay pipes and mug/cups (i'm saying this just in case it effects what i should add to the clay. i have no desire to make anybody sick or hurt them because i used the wrong additive). I have been bitten by the clay bug (HARD) but i've reached an impass. Please help me. Any and all information will be greatly appreciated. i eagerly await your responses. i'm ready and waiting to satisfy this pottery itch. I can't seem to think about anything other than clay now-a-days.
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