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Posts posted by MFP

  1. Replaced the switch and the problem persisted. Fixed it by gluing one inch of refractory blanket to the outside of the kiln bottom. I also have a firing schedule where the bottom is always turned up more than the middle. I also put a final shelf between the last pots and the lid. I end with the bottom of. High, the middle on 5.5 and the top on 6. Cone 6 everywhere now!

  2. On 4/8/2020 at 9:42 AM, desertpotter said:

    Hi there, 

    I have an older manual kiln with three zones, each controllable by a 1/low to 11/high knob. This kiln is run by an updated kiln controller box, which has only one thermocouple in the middle of the kiln. I always have all three manual knobs set to high during firings, letting the controller box run the firing program. 

    The last cone 5 firing I ran had the top most and bottom most shelves under fire, which I believe might be due to the fact that they were short (1-2 inch posts) shelves and the circulation just wasn’t getting to these areas. My question is - can loading the kiln poorly like this attribute to under firing by that much? Is it a good idea to set the manual knobs on the top and bottom sections of the kiln to high, and leave the middle knob on say, 8 or 9? Hope this makes sense! I will also be keeping my shorter shelves in the middle of the kiln since I now know how badly this can effect the firing.

    I also had the idea of slowing down the last leg of the firing schedule, which was at a ramp of 120 degrees per hour to maybe 110 to even out the firing. Thanks for any guidance! 

    I fixed this same problem by doing three things:


    1. I glued one inch of refractory blanket to the outside of the bottom of the kiln.


    2. I always put a shelf over the last shelf under the lid.


    3. I have a firing schedule where the middle is never turned up onn high. I finish with the bottom on high xz the middle on 5.5 and the top on 6. If anything the top got a bit hot.


    Hope this helps


  3. Neil...do you know anything about the big Olympic gas kilns?  Sarah Perez is always after me to get my huge gas kiln operating....but said that my burners might need to be "braised and redrilled".   I have no blasted idea what that means. The kiln has never been fired. I did anticipate lots of quality time with the burners and WD40 to get the ports open.  The burners I had before were home made. I don't know anything about manufactured burners.  These burners were from the original company whose factory burned down.

  4. Thank you, Hulk and Neil. The switch I received is white and the shaft does have a retention spring thing. I will try WD40 and see what results I get with that. I got it just loose enough with a slight pry that I wound up getting it off the calibration and had to make sure everything was lined up correctly again.....and put a piece of sponge behind the switch to keep it from twirling..  And yes, I spied the retention screw behind the knob....undoubtedly somewhere there is a special skinny wrench for unscrewing it.....hence the lock spring on the knob. I want to see how it fires now that the top wire is securely crimped. I will let you know what happens!  My knob is one of the silver ones with  Lo, 1 to 6 and High. 

  5. Yes....I know they are cone 10 clays that will remain open at 6. I was not planning on using them on anything that would hold liquid. But thanks for letting me know!  I also bought some 38 in the past and it also throws very well. Just not as white.  I am thinking that if I finally make myself fire the gas beast, I will use these 2 for my stoneware.  There is an NZ 10 glassy porcelain that is very good but spendy!  They have a cone 10 porcelain called White Rose which I highly suspect is the cone 10 version of Rainier. It is very similar and a toothy porcelain. 

  6. Yes, I finally figured that out....it's amazing what reading the box will do for you!  Yes...I get Rainier from Kyle. I tried a bunch of different porcelains and I like it the best. It actually is more cooperative than the stoneware I bought from CAC.  I finally figured out that If I want to do plates, I better be prepared to dry them for 3 weeks, no matter what I use. That seems to be the trick with all these new clay bodies. I have been trying Amaco Amix   #11. I get it from School Specialty on ebay....their shipping is $5, no matter what you buy and it makes this very white stoneware affordable. 

    Check in on Kyle....it's been a rough road for him lately. 

  7. Thanks Liam....there's nothing in that section except wires an way....once I got it open it was pretty straightforward. The top element wire popped right out when I opened the box.....that and how I was loading it might account for the problem.  I have a new switch but couldn't figure out how to get the darn knob off.  All the  electric guts must be in the center section.  It does have a coil but since the other two sections are reaching hard 6 that isn't the problem. I have learned to cover the last part of the load with a shelf....then I don't have any heat loss through the lid.  I also turn the top and bottom on high and leave the center on 6. That has worked really well....at least in terms of the top. 

  8. Thank you so much Neil. I had thought I was packing it too tight with too short shelves. I took off the cover yesterday to attempt to replace the switch....and voila!  The top wire to the element popped right out....it had not been crimped tightly. So that might have also contributed . It is an old Olympic with knobs. I couldn't not get the knob off and then had to get the switch lined up with the knob again so it displayed the right place on the dial. So the switch was not replaced.  I did do a bisque prior to this where I left the bottom more open....and when it reached red heat, the color was uniform in all three ports.  Also....plug peep holes or leave them open? I was wondering if I was starving the bottom of O2 by putting in the peep. 

  9. I have an Olympic 1827H electric kiln.  The bottom ring was hardly reaching cone 4. Since it is an old kiln, I thought the elements were the problem and replaced them. Fired again. Still cone 4 in the bottom ring. Olympic thinks it is a switch problem and is sending a new one. But I have seen references on here to relays. So I was wondering if you could please tell me more about relays and how to determine if they are functioning properly?  Apparently there is also some kind of coil.  The elements are heating up but just appear to not be getting enough juice. Middle and top rings are achieving hard cone 6, 


  10. Yes....although "talc" is a homogeneous term in pottery meaning "stearite", it is a heterogeneous term in mining....referring to at least 8 different minerals depending on the application.  Pottery was even less of a market share 30 years ago than it is now.....there's no telling what we got when we were using "talc".  I know we had to abandon at least two glazes because one of the materials that showed up were no longer the same....and we couldn't get what we had previously.  I am sure the university was using soapstone....but other people were using whatever people said "talc" was....glaze theory and glaze calculation was not the norm in the pottery community.....you had recipes and you followed them....if a material changed....you just did not have that glaze any more. Thankfully, times have changed. 

  11. This was my point.....that pyrophyllite is marketed as talc because of its physical properties, not its chemical properties. Layered talc often was in alternating layers of talc and pyrophyllite. And I agree.....that the name was not in any way related to the contents. What I highly suspect was that the talc we got was from a layered deposit contaminated with Dolomite, pyrophyllite, and Wallosanite.  However, since my friend has tried all the current magnesium containing talcs.....it didn't hurt to give pyrophyllite a test---just in case that is the "talc" we got 35 years ago--- since these marketers only care about the physical properties in most cases  (for example.....one mineral they call "talc" is almost entirely asbestos).. When you look at pyrophyllite and touch it, you could not tell the difference from true talc. 

  12. After what I  have discovered about the variability of talc deposits and all the different minerals being sold as talc.....I would not be surprised at all,,,,especially considering that the ceramics community was 1% of their market....and physical properties were what they were after, not chemical properties---which don't seem to concern them at all..  Bottom line....pyrophyllite is considered a talc....not the talc we use in the pottery community....but it is used in other ceramic applications.  And like I said....Mark is not out anything to try it in that glaze recipe. 

    I know when I am doing tests of my old glaze recipes, I am going to one of each....because this ugly grey stuff is nothing like what we used to use. I sent what 30 year old talc I had to Mark already, but I only had a couple of pounds left. I had not seen this stearite stuff at that time. I can tell you.....it is not the same as what I used in the past. If pyrophyllite isn't what it was.....we are both screwed in terms of our old recipes. Since Mark had already told me that all the current ones did not work......it was clear that magnesium might be the issue.....and pyrophyllite has hardly any. 

    So...if this "talc" doesn't solve his problem with that one glaze....I have no further suggestions--- as it is clear that the chemical properties change from mine to mine and within the same mine on any given day. 

    I got a lot of info from the US Geological service. That's how I found the North Carolina mines. 

  13. PS---what I also discovered is that although we are interested in the chemical  properties of talc.....the people mining it are much more interested in its physical properties.  The reason they call pyrophyllite a talc is because their physical properties are very similar. I can tell you in handling this powder that it acts exactly like talc....sticks to your fingers etc. 

    Sorry....the digitalfire is not consistent with what I have previously found. I am not going to argue about this any more. I was trying to help Mark out. Since I bought the pyrophillite....he is not out anything to make a 100 gram batch to test it for his one glaze.  I am also talking about chemicals that existed 35 years ago.....and things were much different then.

  14. Thank you.....yes that is what I am trying to duplicate. What I meant...sorry I was not clear....is can you mix a stain with water and use it like an oxide on bisqueware.....and is this what he is doing here? Or is it being added to a thin slip and then applied. I am thinking that Mason best black. There was also a stain in the "adding colorants " manual here on the site. But it looked pretty complicated and had to be milled for hours and I do not have a mill. 

  15. Did you see the above grid that says Pyrophyllite-Talc group?  What I am thinking is that we were sold "desert talc" that was pyrophyllite which is why it is so different in the glazes calling for it==because it doesn't have magnesium. . I am NOT suggesting it is true talc.  I bought this from Ceramic Supply in Pittsburgh.....so someone is using it for something. If it doesn't work for Mark...then it has simply been ruled out. He said something about Texas talc kind of working and that was where pyrophiyllite was being mined before that mine shut down. Before that was the Mojave desert. Now only in North Carolina. I have put hours of research into what "desert talc" was......and discovered it wasn't really talc at all but the industry calls it that. 

    So..it is simply an option for Mark to try, If it doesn't work....then some other kind of screwy talc is the reason. IN researching this, I found that the talc is often mixed with other veins of minerals like Wallosanite, Dolomite, and calcium deposits. So....if the "desert talc" he was getting was contaminated with a particular blend of talc, Wallosanite and Dolomite....we will never be able to duplicate it.....and would account for the difference as the amount of magnesium would be much reduced.  I actually talked to the mine in Montana.....and every day there is a different type of soapstone coming out of the mine.  It changes constantly.  And they don't do any assays to determine its composition ongoing.  So literally, the talc can vary from batch to batch. 

  16. There are far more forms of talc than are used in pottery.....and it is possible that over the years, depending on which mines were operating.....people got sold "talc" of all kinds...not very helpful when trying to replicate old recipes and  vendors have used this generic term for what are distinctly different minerals. The stearite group is what we are using now as "talc". 

    Varieties of TalcHide

    Beaconite A fibrous variety of talc resembling asbestos.
    Chromian Talc A Cr-bearing variety of talc.
    Polyphant Stone A greyish-green potstone flecked with whiteand brown. Used since Norman times as an ornamental stone in churches.
    Pseudolite Octahedral talc pseudomorphs after spinel.
    Steatite A massive variety of talc with a greasy feeling, often used for ornamental carvings.
    Zincian Talc Zn-bearing variety from the "Mixed Series" formation, Nežilovo, Macedonia. Associates, i.a., with ferricoronadite.

    Relationship of Talc to other SpeciesHide

    Other Members of this group:
    Ferripyrophyllite Fe3+Si2O5(OH) Mon.
    Minnesotaite Fe2+3Si4O10(OH)2 Tric. 1 : P1
    Pyrophyllite Al2Si4O10(OH)2 Tric. 1
    Willemseite Ni3Si4O10(OH)2 Mon.

    Common Associates

  17. You are missing the point completely. What I found is what used to be called "desert talc" and was called for in old recipes was identical to pyrophyllte and was NOT stearite,,,,aka soapstone....and is why old recipes that call for "desert talc" do not work with stearite because of the magnesium content. I didn't name it that....someone else did. It is not a rock like soapstone and is deposited in layers like shale.  So we will see if it replaces the misnamed desert talc in Mark's recipes....and it is likely to be what I will need in my old recipes. 

    Here are the four types of talc.....number three is the pyrophyllite group. What is used now is the stearite/magnesium group. In the past, it was different,.

    There are four types of talc deposits:
    • Deriving from magnesium carbonates. ...
    • Deriving from serpentines. ...
    • Deriving from alumino-silicate rocks. ...
    • Deriving from magnesium sedimentary deposits.

    The type called desert talc is only 10 percent of all talc produced....most likely because it is SO different from the others.

  18. Maybe in CURRENT recipes....but I am talking about people who are having trouble with OLD recipes because this new talc has those impurities in it.  OLD recipes rely on the OLD talc.  This talc is used for a different purpose.



    PYRAX® B Pyrophyllite is used in rapid-fire ceramic wall tile bodies. It lowers firing temperature; produces low moisture
    expansion bodies with good craze resistance; increases thermal shock resistance; and greatly increases firing strength in
    vitreous bodies. PYRAX B promotes the development of mullite when substituted for an equivalent amount of feldspar or
    PYRAX ABB Pyrophyllite is generally used for the same applications as PYRAX B, but it shows wider variations in mineralogy
    and particle size distribution.
    PYRAX RG refractory grade pyrophyllite is used in refractory mold washes insulating firebrick, metal pouring refractories,
    alumina-silica monolithic refractories, ramming mixes, gunning mixes, castable mixes and kiln car refractories.
    Al2O3 17% 21% 0. 7% 38% 36%
    SiO2 77% 72% 50% 45% 46%
    Fe2O3 0.4% 1.9% 0. 2% 1.0% 1.7%
    TiO2 0.2% 0.5% < 0. 1% 1.5% 1.4%
    CaO <0.1% 0.1% 45% <01% <0.%
    MgO <0.1% 0.1% 1.5% 0.2% <0.1%
    Na2O 0.2% 0.4% < 0.1% <01% <01%
    K2O 1.2% 0.3% < 0.1% 0.5% 0.4%
    LOI 3.6% 4.0% 1.6% 14% 14%
    For technical questions regarding these products or their use in your application, please contact our R&D center at
    MineralLab@vanderbiltminerals.com or click on the Technical Inquiry link on any www.vanderbiltminerals.com website page.

  19. Two different minerals with similar physical properties are talc and pyrophyllite. Their physical properties are nearly identical. Both are very soft: talc is the softest mineral on the Moh's hardness scale at 1, and pyrophyllite is 1 to 2.Both talc and pyrophyllite have perfect cleavage in one direction. This means that these minerals break into thin sheets. As a result, both feel greasy to the touch (which is why talc is used as a lubricant). They are both formed in metamorphic environments as the result of changes in silica-rich dolomite. 

    Steatite and soapstone are impure, massive forms of talc that lack the distinctive cleavage mentioned above."   These are the forms that are currently being sold to Laguna.  This is not the same as pyrophyllite.

    So maybe talk to the Laguna guy about getting some North Carolina talc. The chemical formula for the pyrophyllite is :

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