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Magpie

Pinging Pots

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Hello,

 

I recently set up a small home studio and this weekend was my first glaze firing. I didn't rush...did a slow galze fire to cone 6 (with an L&L pre-programed firing schedule) and let is cool for 24 hours. The pots were just barily warm when I took them out of the kiln. I used trina buff clay from Highwater (rated for cone 6-10) and Amaco Potter's Choice glazes (rated for cone 5/6). The pots look nice but they are "pinging" a little (these are functional pieces...mugs, bowls, etc.). No cracks are visible. Is this normal?

 

My previous throwing experience has been in teaching studios were students do not load or unload the kiln or having anything to do with firings. So before, I would glaze a pot, put it on the shelf, and pick it up in a week. So, I've never heard the "pinging" before.

 

Thanks for your thoughts!

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I've noticed this too but I haven't been able to figure out which ones are doing it.

Doesn't seem to hurt anything. You getting any crazing?

I also use Highwater stoneware clays and Amaco Potter's Choice glazes.

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The pots don't seem to be crazing...at least visually I can't see it. I read to fill the pots with water and put them on a dry surface to see what happens. I did that and a few of the pots are seeping water. Which, leads me to believe that there are small cracks in the glaze.

 

So, what does that mean? Besides a damp table, are my pots not functional? I've read differing opinions on this.

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Hello,

 

I recently set up a small home studio and this weekend was my first glaze firing. I didn't rush...did a slow galze fire to cone 6 (with an L&L pre-programed firing schedule) and let is cool for 24 hours. The pots were just barily warm when I took them out of the kiln. I used trina buff clay from Highwater (rated for cone 6-10) and Amaco Potter's Choice glazes (rated for cone 5/6). The pots look nice but they are "pinging" a little (these are functional pieces...mugs, bowls, etc.). No cracks are visible. Is this normal?

 

My previous throwing experience has been in teaching studios were students do not load or unload the kiln or having anything to do with firings. So before, I would glaze a pot, put it on the shelf, and pick it up in a week. So, I've never heard the "pinging" before.

 

Thanks for your thoughts!

 

 

 

Not all clays and glazes are compatible. The PC glazes are beautiful I am also planning to try them. However, I am going to test first on one small textured vessel and I am using Amaco clay.

 

I would advise anyone if you are planning to try a manufactured product such as glazes or underglazes use the same manufacturer’s clay. The Amaco PC color samples were fired on #65 Porcelain and the PC mixed color samples were fired on #38 White Stoneware with special brushing methods. (There is a brochure available on the Amaco website).

 

I am not saying that your results will be perfect but you stand a better chance of not having defects if you use compatible clay.

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If you hear pinging it means your glaze does not fit the clay and it is cracking.

That's what the noise means ... Sorry!

 

Does not matter how slowly you fire ... they dont fit.

Size 8 jeans on a size 12 butt .... something's gonna give.

 

Glaze fit is hard ... It takes time and testing to find a perfect match but don't settle for your first try ... don't give up.

Try another glaze if you love the clay or try another clay if you love the glaze.

 

The folks at Highwater are great and I am positive they can steer you to better fitting glazes ...call them tomorrow and ask.

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Pinging pots means crazing but can the work be used?

 

It depends ... the number one answer in clay!

 

If the work is decorative it doesnt really matter unless it's going outside where water absorption and temperature changes could lead to cracking in winter.

 

If it's on PERSONAL use serving pieces then just be careful how you use it and wash it ... don't put it in the microwave right out of the dishwasher ... Don't use it in the oven for baking.

 

I WOULD NOT SELL piece that pinged or crazed to anyone else for functional use. That sound means something is going on and it could be more than just crazing ... Handles could fall off ...the whole piece could crack. Also, not a safe piece to give to someone who might not realize the limits of pottery.

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The pots don't seem to be crazing...at least visually I can't see it. I read to fill the pots with water and put them on a dry surface to see what happens. I did that and a few of the pots are seeping water. Which, leads me to believe that there are small cracks in the glaze.

 

So, what does that mean? Besides a damp table, are my pots not functional? I've read differing opinions on this.

 

 

For pots to hold liquid they need to have a water tight clay body. You need to make some pots without glaze and fire them the same way you fire your glazed pots. Fill these with water and see if they seep. Better yet, do an absorption test. Pull the pot out of the kiln without letting it cool completely, you want it above 212 F. weigh it to the nearest gram, then boil it for 2 hours. Pull it from the boil, let it sit and steam off until dry then weigh again. See how much weight it has gained. You hope it has gained none. Then let it sit in the water until luke warm and repeat. Calculate the % change and that is the absorption rate for that clay when fired in your particular kiln.

 

Stoneware clays should be less than 3% absorption and as close to 0% as possible without melting or slumping in the kiln.

 

After you figure out if your clay/firing will hold liquid, THEN you can find a glaze to fit it.

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One other thing. You need to put witness cones in the kiln to test the computer. Just because the kiln thinks it is up to temp, doesn't mean it is right. There is also the heat work that you need to consider. Just because it has reached a certain temperature, that doesn't mean enough heat has gone into the pots. Your pots may be underfired.

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One other thing. You need to put witness cones in the kiln to test the computer. Just because the kiln thinks it is up to temp, doesn't mean it is right. There is also the heat work that you need to consider. Just because it has reached a certain temperature, that doesn't mean enough heat has gone into the pots. Your pots may be underfired.

 

 

That's what I am wondering about, Ben. If she says they are seeping liquid, doesn't that mean the clay isn't vitrified, a 6-10 is a wide range, somewhere in there is the temp for vitrification ??

Maybe the problem is more basic that glaze fit?

 

 

Highwater makes ellen buff and desert buff, both are ^5-7 range and good throwing clays, maybe you could try one of them for the ^6 firing and glazes. I use them often and have no problems with other ^6 glazes.

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Chris.

 

Recently I was helping the instructor unload a (cone 10 gas-fired reduction) kiln with other students at the college where I do my work. The pots were still hot enough that you couldn't hold them for more than several seconds, and the shelf we unloaded to was in a cold (and windy at that point) part of the kiln yard. It was the first time I've heard so much pinging (although I would have referred to many of the sounds I heard as outright "cracking"). The instructor said that the particular pieces that were pingingcracking most loudly were a clay that was notorious for having glaze fit problems. I don't know anything about firing and unloading kilns correctly, but it seems to me that it could also have been the quick temperature change from the kiln to the cold atmosphere. Based on this info, what do you think?

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Well, just from what you describe, I would not think it was a great idea to unload too hot pots onto a cold surface.

I would venture that kiln unloading had to fit into a busy schedule and that was the only time it could be done?

Most especially at year end, firings at schools can get fast and furious because everyone wants their pots fired yesterday.

Instructor was most likely getting ready to load up again. That's just what happens sometimes.

 

This is probably the number one joy of getting your own kiln ... control over your firings.

I would also reserve judgement on that clay body until I had done some tests myself

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Thanks for all the responses.

 

I've done a lot of reading about clay bodies over the past few weeks. So, from what I've read, the clay manufacturers give a wide range of firing temps which may not be completely valid. I was throwing trina buff, a cone 6-10 clay. It's not that this clay can't be fired in that range but that cone 6 is at the low end of the range. Also from reading, I've learned that each clay body has a specific temperature at which it vitrifies. So, firing trina buff to cone 6 may not get it to a cone where it completely vitrifies. The recommendation that I read said that you should choose a glaze you like, say a cone 6 glaze, and then pick a clay body with that cone at the high end of the firing range. So in this example, I would pick a mid-fire stoneware with a range of cone 4-6. This seems to make since to me...any thought on this?

 

So I was thinking for the next firing I will try a cone 4-6 clay with the same glazes and see what happens. I'll also add in a few naked peices to test for absorption. And, some cones to test my controller.

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When I look for a clay body, I look at four things: color, firing range, shrinkage, and absorption. For functional ware, e.g., vases that will hold water, bowls that might find their way into a microwave and/or dishwasher, I look for a claybody with a low absorption percentage at cone 6 -- my usual firing temperature. In looking at the Highwater website, they indicate Trina Buff has a range of 6 to 10; at cone 6, it shrinks at 11% and has an absorption of 1%; at cone 10, it shrinks at 12% and has an absorption of less than 1 percent. My guess is Trina Buff is pretty vitrified at cone 6 given that low of an absorption rate. For comparison, consider Standard's 153 stoneware; it too has a firing range of cones 6 - 10; however, at cone 6, the shrinkage is 11.5% and absorption rate 5%, while at cone 10, shrinkage is 13% and absorption is 2%. This clay body is more vitrified at cone 10 than cone 6.

 

I believe glaze fit has more to do with the expansion/shrinkage compatibility of the clay body and glaze than whether the clay body is vitrified or not. My guess is the difficulty you had using Trina Buff and Amaco PC glazes lies more in expansion/shrinkage incompatabiity and is not an issue of clay body vitrification. It is possible you could still have a glaze fit problem with a cone 6 clay and the Amaco PC glazes.

 

A good explanation of glaze fit/durability is in the Hesselberth and Roy book, Mastering Cone 6 Glazes. In my opinion, start with a clay body that works for you and what you are making and then build your glazes around that clay body.

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So, the trina buff clay should have been vitrified at cone 6. Pinging pots aside, the only reason that water would seep through would be that the fired clay body was not holding water (possibly under-fired)?

 

I know that stoneware for functional use should be 3% with the goal of being closer to 0. But even at 3%, will pots seep liquid? Would they be microwave/dishwasher safe?

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Regarding dishwasher safe, I think in terms of glaze durability . . . is the glaze finish durable/mature enough to withstand multiple washings? You don't want a glaze finish that will dull or become discolored. Besides not looking nice, that could lead to oxides/colorants leaching into food items.

 

Microwave safe . . . any clay body that absorbs water could have a failure in a microwave. Microwaves heat by exciting molecules to generate heat; the microwave does not differentiate between the food item and the vessel holding that item. For water absorbed in a clay body, the water molecules are excited, generating steam and water vapor; and, we know steam and water vapor expand when heated. As a result, the steam/vapor could exploit a weakness in a seam or a more narrow wall, causing the vessel to crack or break. I caution customers not to take a vessel (bowl, cup, platter, etc) directly from a dishwasher (or refrigerator) and heat it in a microwave or oven. And, for functional ware, I try to minimize the amount of unglazed surface to reduce the opportunity for water absorption. For expample, I ofen put feet on platters so I can glaze the bottom of the platter. That also helps keep food warmer since the platter sits above the table, not on it where heat transfers to the table.

 

It is my understanding that unless a vessel is completely vitrified (0% absorption), it has the potential to absorb and seep water. How much water is leaked depends on the percentage of absorption and the weight of the vessel. An underfired clay body would be more porous than a properly fired clay body that reaches maturity. A compatible glaze that fits the clay body helps; one that is not compatible, e.g., crazes, could allow water to pass through and be absorbed into the clay body.

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Magpie,

 

Chris is right when she says 'Glaze fit is Glaze fit'. Our firing schedules can affect the relative shrinkage of a glaze or clay body but, at the end of the day, some glazes just DON'T fit some clays period.

 

More importantly however, in regards to food safety, is the physical presence of cracks or other imperfections in the glaze. If a glazed pot leaks water, then it has cracks in the glaze somewhere; even if you can't see them. An unglazed surface poses a harder problem for testing. EVEN a completely vitreous surface may not be food safe, depending on the physical surface structure of the clay itself. Pits in the clay (even VERY tiny ones), grog particles, or other breaks in the smooth surface can render a vitreous pot unsafe. India ink or VERY strong tea will usually stain those cracks and pits and make them visible to the eye, but a pot that leaks water makes that testing step irrelevant. If water can go there, you can bet that bacteria (which is considerably smaller) will go there as well.

 

ANY pot that has a crazed glaze, is no longer considered food safe by federal health standards REGARDLESS OF THE REPORTED SAFETY OF THE GLAZE ON THE LABEL. The problem here is not one of a glaze that is safe chemically, but a food-contact surface that has been rendered UNSAFE by a physical defect. The cracks provide bacteria a place where it can hide and multiply. Soap and commercial sanitizers must come into direct contact with bacteria to kill it, and the soap molecules may not be able to get into the cracks where the bacteria is hiding. The heated drying cycle on most modern commercial dishwashers DOES get hot enough to kill many bacteria strains; but you are assuming your customer has a modern dishwasher that is working properly.... There are simply too many variables that are out of your direct control. That is why health and safety standards require a glazed surface to be free of defects; and all commercial glazes that are labeled as food safe also carry the disclaimer that they are only food safe when "fired properly to maturity".

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Kiln doc,

 

I'm not much of a potter, but you are overstating your case. Bacteria are not smaller than water. Bacteria may be able to go where water can go, but bacteria can be seen under a microscope. Water molecules are still invisible at that magnification. A number of water filtration techniques rely on the fact that Bacteria are larger than water molecules. Please, let us not get ridiculous here by misstating the problem. If you pot leaks and it's meant to be functional, that's always bad. But if it doesnt leak, and has a crackle glaze, can we say reliably that it's safe or unsafe?

 

Moreover, I wonder what federal health standards you are citing. That would be really instructive, and end this crackle glaze debate. I can find some standards for commercially prepared foods (FDA has some sealed container requirements, for instance), and for food preparation areas (USDA for sealed concrete in meat packing plants); but I can't find any for ceramic tableware. So, where would I find the federal standard you are talking about? I've bought crackle glazed wares from JC Penny that it clearly believed were food safe.

 

I hate to say it, but I have a feeling that glaze manufacturers say what they do because they are afraid of the possibility of litigation, rather than because of demonstrated evidence of dangerous defect. Maybe professional potters should all follow their lead, but professional potters also ought to be informed the of the relative risks if their glaze develops crazing after it has left their hands. Hence, this discussion I think.

 

Magpie,

 

Chris is right when she says 'Glaze fit is Glaze fit'. Our firing schedules can affect the relative shrinkage of a glaze or clay body but, at the end of the day, some glazes just DON'T fit some clays period.

 

More importantly however, in regards to food safety, is the physical presence of cracks or other imperfections in the glaze. If a glazed pot leaks water, then it has cracks in the glaze somewhere; even if you can't see them. An unglazed surface poses a harder problem for testing. EVEN a completely vitreous surface may not be food safe, depending on the physical surface structure of the clay itself. Pits in the clay (even VERY tiny ones), grog particles, or other breaks in the smooth surface can render a vitreous pot unsafe. India ink or VERY strong tea will usually stain those cracks and pits and make them visible to the eye, but a pot that leaks water makes that testing step irrelevant. If water can go there, you can bet that bacteria (which is considerably smaller) will go there as well.

 

ANY pot that has a crazed glaze, is no longer considered food safe by federal health standards REGARDLESS OF THE REPORTED SAFETY OF THE GLAZE ON THE LABEL. The problem here is not one of a glaze that is safe chemically, but a food-contact surface that has been rendered UNSAFE by a physical defect. The cracks provide bacteria a place where it can hide and multiply. Soap and commercial sanitizers must come into direct contact with bacteria to kill it, and the soap molecules may not be able to get into the cracks where the bacteria is hiding. The heated drying cycle on most modern commercial dishwashers DOES get hot enough to kill many bacteria strains; but you are assuming your customer has a modern dishwasher that is working properly.... There are simply too many variables that are out of your direct control. That is why health and safety standards require a glazed surface to be free of defects; and all commercial glazes that are labeled as food safe also carry the disclaimer that they are only food safe when "fired properly to maturity".

 

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I have been reading this dsicussion with great interest. I never had any pinging with my work when I used Standard 306 or Highwater Red Rock clay with Potter's choice and Coyote glazes. When I used Highwater Little Loafers, and the same glazes everything pinged. I didn't know what was going on! All clays say they are cone 6, as do the glazes. They were fired to witness cone 6 with a slow cool down. I don't see any cracks but guess I will have to do some tests since most of my work is functional.

Thanks for all the good information. (On further investigation, Coyote Ice Blue crazes when used on Little Loafers clay. Potter's Choice glazes were fine.)

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Potters have been debating the crackle/bacteria debate forever it seems.

People are much more likely to get sick from the food than from the dish.

I think this whole bacterial growth in crackles has never been scientifically studied but is a true sounding cautionary tale.

Like the Halloween candy scares ... It just keeps being repeated without any evidence.

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@ LawPots:

 

First off, I was mistaken. Water molecules are indeed considerably smaller than bacteria. I apologize for my mistake on that point and stand corrected.

 

I also believe you make a good point about getting crazy by misstating the problem. My post was intended to illustrate that there are physical defects in addition to chemical ones, that can render a glazed surface not suitable for food contact. If we sell our work to the general public as 'food safe', then we also have a legal responsibility to be reasonably certain that it actually is food safe.

 

As far as the legal code I refer to:

 

The US FDA ‘Food Code’ contains the provision describing the acceptable design and construction of Food Contact Surfaces. While initially intended as governing the retail food service industry, it is also used, albeit broadly, as a supporting legal definition within the realm of Consumer Protection as to the appropriateness of an object to be marketed as safe for food contact. It is substantively different from ASTM D 4236 (which is the chronic hazard labeling standard in the US), and the U.S. Labeling of Hazardous Art Materials Act (LHAMA).

 

According to the United States Food and Drug Administration, 49 of the 50 US states have adopted food codes patterned after the FDA food code (from various years). The single state that has not adopted the US food code, is North Carolina, which chose instead to pattern its local food code after a 1976 Model Foodservice code instead.

 

I will Cite the applicable portion of the FDA 2009 Food code below (you may find the full text of the code on their website www.fda.gov):

 

4-202.11 Food-Contact Surfaces.

 

 

  1. (A) Multiuse food-contact surfaces shall be:

 

  1. (1) Smooth; Pf
  2. (2) Free of breaks, open seams, cracks, chips, inclusions, pits, and similar imperfections; Pf

 

Now I agree with Chris that the debate over cracks and bacteria is contentious, and I too believe that a person is more likely to get sick from improperly handled or prepared food than a crazed glaze. Further, I also agree that manufacturers are quite conservative and risk-averse when it comes to public health and safety but, then again, isn’t that a good thing? Don’t we want the manufacturers of the products we buy to err on the side of safety? Since we are also manufacturers, shouldn't we hold ourselves to those same standards?

 

In conclusion (and I apologize to all for the book-sized post), let me try to restate my argument:

 

 

  1. A debate on this subject continues to go unresolved.
  2. The more conservative (safer) response is that “a crazed glaze is not appropriate for food contactâ€
  3. A legal definition exists that supports the conservative response.
  4. It is possible and practical to produce functional work that is not crazed, and market that product as food safe.

Therefore, it is both logical and practical to adopt the conservative attitude until/unless the legal definition changes (in response to the debate) to reflect new and conclusive data.

In other words, it is best to consider a crazed glaze to be NOT food safe until science proves otherwise.

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Kilndoc,

 

That's a good find, and for me it settles the debate. Don't sell pots that have crazing on the food surface - too risky for me. I'll also note there's a section on Utensils in the food code that's separate from your cited section:

 

Multiuse

 

4-101.11 Characteristics.

Materials that are used in the construction of utensils and food-contact surfaces of equipment may not allow the migration of deleterious substances or impart colors, odors, or tastes to food and under normal use conditions shall be:

A Safe;

B Durable, corrosion-resistant, and nonabsorbent;

C Sufficient in weight and thickness to withstand repeated warewashing;

D Finished to have a smooth, easily cleanable surface; and

E Resistant to pitting, chipping, crazing, scratching, scoring, distortion, and decomposition.

 

Whatever the state-sanction is for ignoring these codes, potters should be concerned about the effect of this code on tort law. If a customer got sick drinking or eating from a crazed pot, the potter could easily be on the hook for selling a defective product. It's little wonder that commercial sellers do that warning about crazed glaze. Now the question would be . . . If you sell a decorative teacup with a crazed glaze, and your warning is removed or ignored - are you still liable if the customer ignores you? Huh, it's bad being a lawyer.

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