PeterH
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Posts posted by PeterH
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3 hours ago, HenryBurlingame said:
I’m still having a hard time finding good cone 6 clay bodies. Does anyone know of a company that uses frit in their clay bodies to get full vitrification at cone 6? The clays I have tried have so far had high or uneven absorption, and if they do have good vitrification they reclaim weird (like they are getting short for some reason or something). I don’t want to HAVE to make my own clay bodies but I am starting to think about it lol.
Would tweaking the reclaim be easier than making your own body (which might also have reclaim issues)?
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On 8/19/2024 at 5:19 AM, Bubba said:
Beauty model AB18 that I just picked up has 4 toggle switches instead of those familiar timer dials.
... my emphasis.
I'm not an expert but I think you've got your mental model badly wrong.
- You don't have a timer dial because your kiln-sitter doesn't have this safety "watchdog timer" feature.
- The 4 toggle switches are serving a similar function to the rotary "power" switch(es) usually used in conjunction with a kiln setter.To elaborate, typical firing instructions are given on p10 of
INSTRUCTION MANUALFORA4HKT AND A8HKT
https://cressmfg.wpenginepowered.com/wp-content/uploads/A8-A4-inst-manual.pdf
These models are equipped with an infinite power control switch that is used to increase the power level during firing. Thick walled or hand made pieces should be fired more slowly. A typical firing schedule for firing hand made pieces 1 or 2 hours on “1” or low, then 1 or 2 more hours on “5” or medium, then turn to high for the remainder of the firing.1) You don't have a rotary dial, instead you have 4 switches. Are they labeled? If not, my first guess is that they control the power going to the 4 kiln elements. (Your picture of the kiln interior suggests to me that there are 4 elements.) I assume you turn then on progressively, but haven't a clue what order to use. (This has to give a spatially more uneven heating pattern that using an infinite switch.)
2) The end of the firing could be determined by looking at pyrometric cones through the peephole. But the kiln-setter automates this by "looking" at a small cone placed inside it, and cutting off the power when it softens.
3) However cone-sitters sometime stick, so some have a watchdog timer. You set this to a time a longer than the firing should take. If the cone-sitter sticks the timer will cut of the power - hopefully before too much damage is done. You might like to set an alarm clock to remind you to check the kiln has shut itself off.
PS Can you say what the model-number of your kiln-sitter is, so we can try to find a manual for it. Meanwhile here is the manual for models LT-3 & LT-3K.
http://www.fireright.com/docs/kilnsitter/lt3andk.pdf -
19 hours ago, SFKeller said:
I was installing elements in the last ring and suddenly got paranoid that the previous person did it correctly.
It's always a good idea to cross-check if you can.
With the kiln unpowered (and hence relays open) you might measure the resistances between the terminals on the contact block.
The element resistances are given in https://skutt.com/pdf/service_manual/11_4.pdf
MODEL VOLT PHASE #1 #2 #3 #4 #5 #6
KM1227 240 1 13.6 13.6 16.3 16.3 13.6 13.6The circuit diagram shows the two elements in each ring are wired in parallel, so the resistances should be:
[1]->[2] = 13.6/2 = 7.8 ohms
[3]->[4] = 16.3/2 = 8.15 ohms
[5]->[6] = 13.6/2 = 7.8 ohms- Bill Kielb, iffetorbay and SFKeller
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Looked up my favorite book on casting. The only thickness reference that I could find quickly was that cottle-boards should 1.5" from the nearest point of the master.
The book's cover reminded me that you need to have room for any natches.
Nice book, but not cheap.
https://www.bookfinder.com/search/?full=on&ac=sl&st=sl&ref=bf_s2_a1_t1_1&qi=xMl.V.Jx,EhL0xQKd7,WfD3S9,E_1723980007_1:7148:15334- Chilly and KID-IN-CLAY
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On 8/17/2024 at 7:56 AM, KID-IN-CLAY said:
what is an appropriate thickness a plaster mold must be to work properly?
Should have said that a very thin mould will definitely need to be dried out before making another casting. A thicker mould may allow you to make multiple castings before having to dry it out.
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On 8/16/2024 at 11:51 AM, annika77 said:
my mugs are warping even inside the mold already!!
I'm finding that difficult to picture, could you post a photo?
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5 hours ago, KID-IN-CLAY said:
what is an appropriate thickness a plaster mold must be to work properly? i
I suspect that the wall thickness of the intended casting is-- more or less -- the absolute minimal thickness of the mould walls. (In order to absorb the water from the slip to form the walls.)
The mould also needs to be strong enough handle the weight of the slip needed to fill it, and the handling involved during casting -- perhaps requiring thicker walls Sometimes some extra plaster makes the mould tidier/easier to cast, handle or store.
@High Bridge Pottery's 2-3cm sounds typical for few moulds I have made.
You may find this thread of interest.Note the concerns about the texture 3D printing can produce.
- KID-IN-CLAY and Bill Kielb
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Starting from the idea that making your own "white" magma glaze might be the way to go ...
I've recently started playing with SC, my first test only used 3.4% SC and fired to ^6 it was white. (There's also a bit of green glaze showing at the top). Using more SC has just given me a nasty black result. I would try reducing the amount of SC.
Wondering what might cause the darkening I first remembered
https://community.ceramicartsdaily.org/topic/30023-adding-granular-feldspar-or-stone-to-clay-body/?do=findComment&comment=226744
... where decreasing the SiC grit size tended to be associated with both a better magma effect and darkening
After some sercing (most info seems to be behind paywall) I found:
https://www.sciencedirect.com/science/article/abs/pii/S0272884221028777
The experimental results showed that in SiC powders prepared using the Acheson process, the contents of O, free-Si, free-C, and Fe impurities are high, and those of other trace impurity elements follow the order: Ti > Al > Ni > V.No idea if "our" SiC is produced by the Acheson process, but iron seems plausible as a darkening agent. (And as I understand it most of the fine SiC used in pottery comes from lapidary sources, which are probably not too concerned with impurity levels.)
Confirmation:https://www.arrow.com/en/research-and-events/articles/silicon-carbide-the-future-of-power
Darker, more common versions of silicon carbide often include iron and carbon impurities, but pure SiC crystals are colorless and form when silicon carbide sublimes at 2700 degrees Celsius.So I'm gradually coming round to the idea that covering a gray magma with a white compound has it's merits!
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Loosely related threads?
... I had my new electric kiln "professionally" wired in the garage and was assured all was fine ... went outside during my first high firing to find fire ripping up the wall, following the wires.
- Rae Reich and Bill Kielb
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4 minutes ago, High Bridge Pottery said:
These squirrel hair brushes are the best. They do cost a lot but 100% worth it in my opinion. The big one I have holds enough glaze in 1 dip to do a full coat on the outside of a mug.
Winsor & Newton?
https://www.winsornewton.com/row/brushes/watercolour-brushes/professional-watercolour-squirrel-brushes/ -
8 hours ago, karend5 said:
For a long time I had all kinds of trouble getting my purple glaze to come out purple. Can anyone relate?
Everyone kept telling me it was the clear glaze that it couldn't have zinc in it. I spent all kinds of money trying to find a glaze that wouldn't make my purple blue to no avail. Then I started having other problems and found out I was packing the kiln too tight. I've been putting more space between shelves. I couldn't believe it, my purple came out purple! It was lack of proper oxygen that caused the purple to turn blue! Hope this will help someone.In response to the question "Can anyone relate?"
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Another bright gold 8% in 2g bottles
https://www.dogwoodceramics.com/product/mayco/mayco-overglazes.htmAgain surprisingly little discount for bulk .
1 bottle for $32/gm
12 bottles for $29.50/gm
122 bottles for $28.50/gm -
a) What is your location (what continent/country would you like your suppliers to be located in)?
b) A quick google finds https://www.pottersconnection.co.uk/shop/19/index.htm
10% bright gold: 5gm at £72 & 50gm at £597 so only a 19% reduction for 10x quantity in this case
... I assume they would give you a quote for larger quantities, it might be helpful to have a price-point as you search furtherc) I hope you've read all the H&S advice. For example: Johanna DeMaine https://demaine.org/wp-content/uploads/2022/04/health-safety-and-overglaze.pdf
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Loosely related
Outdoor Weather Resistant Ceramics
https://digitalfire.com/article/outdoor+weather+resistant+ceramics
... including a picture of a terracotta tile half coated with a sealer.
I've no idea how these sealers stand up to repeated cleaning. I wonder if any of the anti-graffiti sealers are suitable.
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On 7/30/2024 at 7:21 PM, Cristobal said:
They seem to increase their porosity while in the kiln. The bottoms of the bricks absorbed my kiln wash on the bottom (hottest) shelves.
Does anybody have an explanation for that? Isn't kiln wash is supposed to be refractory, so it shouldn't be absorbed. Unless the brick is softening sufficiently for the kiln wash to be simply stuck onto it.
What happens if you adopt Rae's suggestion of supporting the bricks off the shelf?
On 7/30/2024 at 7:21 PM, Cristobal said:The glaze seems to be absorbed by the brick, leaving only very thin matte layer. The same conditions create proper glossy glaze on firebricks.
Random thoughts.
1) If all else fails might a brick sealant be an alternative?
2) Would adding gum to the glaze manage to keep more of the unfired glaze ingredients on top of the highly porous surface of the brick? Hoping that the high viscosity of the melted glaze will then keep it at the surface during the firing.
PS Terracotta might not be the easiest body to glaze, especially when not chosen/fired for pottery purposes.
Bubbles in Terra Cotta transparent glazes. What to do?
https://insight-live.com/show_post.php?pz=duwdejucazTerra Cotta
https://digitalfire.com/glossary/terra+cotta
... perhaps especially How to decide what temperature to fire this terra cotta at? -
Perhaps of marginal interest
Sodium silicate stretch ...
Using a Sodium Silicate Pottery Technique to Create Cracked Texture
https://ceramicartsnetwork.org/daily/article/Deliberate-Cracks-Heating-and-Stretching-to-Create-Crackled-Texture-on-Pottery... with terra sig
https://clayartcenter.net/mica-terra-sigillata-product-guide/
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18 hours ago, neilestrick said:
Getting 226 volts instead of 240 is a really big deal for a kiln.
To emphasize Neil's point.
Power is V^2/R
Supplying 226V rather than 240V reduces the power to (226/240)^2 = 88.7% of its original/design value.
For comparison, elements are usually swapped out when their resistance increase by 10%, which reduces the power to 1/1.1 = 90.9% of its original/design value. So a >10% loss of power would be a really big deal.
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1 hour ago, yinjuehua said:
I will look into it further and might end up using ITC 100 to be safe.
You may be interested in this thread & its references
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1. Far more detail on carbon-nanotube based black paints than any of us can handle.
https://www.paint.org/wp-content/uploads/2021/09/Carbon-Nanotube-based-Black-Coatings_Mar-2021.pdf
I think it clearly indicates that this class of material is in a different league from anything you will be able to achieve with a glassy glaze.2. Although it won't be very robust, it might be fun to glaze a teacup with a black glaze and then paint the outside with Black 4.0.
- there are several options for handling the outside of the cup
-- leave unglazed then paint with 4.0 (maybe over an acrylic undercoat)
-- paint the 4.0 on top of the glaze (maybe over an acrylic undercoat, perhaps one sold for acrylic-on-glass work)
-- maybe leave a lip-ring of untreated glaze (H&S, wear, etc)
- Looking at how stained my tea mug gets (and how difficult it is to clean) I don't think it's sensible to try and paint the inside of the cup.
3. In some ways Black 4.0 is reminding me of day-glow colours. Both are, I suggest, so dramatically eye-catching because the human visual system has difficulty assessing the reflectance of the surfaces (almost zero for 4.0 and "greater than 1" for day-glow colours). The day-glow phosphors convert UV light to a narrow visible band, so more light is emitted by the object in that waveband than hits it.- Kelly in AK, okrish and Bill Kielb
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Old threads on ITC
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1 hour ago, HenryBurlingame said:
Interesting. Less than 1% would basically eliminate most cone 6 stonewares all together… nearly every single one from the suppliers in the same state as me (seattle pottery supply and clay art center) are listed as 1% and over.
Which clay are you using?
A quick glance at Seattle suggests that they are giving absorption at mid-range rather than at cone 6, with the odd one <1% at cone 5.
Digitalfire gives information on doing your own shrinkage tests. Maybe worth trying at cone 6 and "extended" firing schedules.
https://digitalfire.com/test/shabPS https://seattlepotterysupply.com/pages/water-absorption-in-pottery-clay
When you are making dinnerware (plates, cups, bowls) that will come in contact with food you would want a clay with the lowest absorption rate possible(<.5%) This is because clays with a low absorption rate will not absorb water even if there is a bad fit between the clay and glaze (cracks in the glaze).When making work that will hold liquids but not necessarily food (vases, bird baths, fountains) an absorption rate can be slightly higher (.5-3%). While the clays have a higher absorption rate they should not absorb enough water to seep through the piece onto the table.
Might be worth asking them which bodies -- when fired to cone 6 -- meet their recommendations for dinnerware.
- HenryBurlingame and Hulk
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Absorption for functional ware
in Clay and Glaze Chemistry
Posted
I'll definitely leave that to @Min.
However in the posting I referenced she does say:
... but I would test the reclaimed batch for absorption in case it also needs a top up of flux.
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BTW it is common to use nepheline syenite as a body flux.
So maybe your problem is partly loss of neph sy in the reclaim cycle.