Guest LibraryKrystal Posted January 30, 2012 Report Share Posted January 30, 2012 Hello! I have been making pottery for less than a year and have a lot to learn. I'd like to get this community's input on a problem I'm having with handles that keep breaking. My favorite handle shape is on a couple of mugs I've made in the attached photo, which also shows a mug that used to have that kind of handle (now broken off). The handles aren't breaking right at the connection point, but rather about a quarter-inch into the handle. This has happened to several mugs with this specific handle design (the broken one in the photo broke closest to the connection point). It takes only a mild clink with another dish from 2 inches away in the sink to break a handle off! Another thing to mention is that all of these mugs use the same clear glaze (sometimes in combination with other glazes) and it regularly has the fine, consistent crazing throughout. I've been assured by other potters at the studio that the crazing is not the problem and shouldn't make anything weaker (and that in this case it is caused by people opening the kilns while they're still too hot). Any input on why these break so easily would be greatly appreciated! I love this handle shape and it fits more comfortably in hand than any other I've tried, so I'm super bummed that they're not as strong as I think they should be. I make the handles with an extruder and handle them carefully. I trim my mugs as soon as I can safely do so, so they're still on the moist side of leather-hard when I attach the handles, which stay on very well through the rest of the process. I am wondering if bending the handles as sharply as I do (the pinched part, which I do slowly) compromises the integrity of the handle... or cutting the attachment ends at an angle with a knife perhaps stretches the clay more than a straight cut would? I'm so confused! Thanks for your help! Link to comment Share on other sites More sharing options...
Pres Posted January 30, 2012 Report Share Posted January 30, 2012 Hello! I have been making pottery for less than a year and have a lot to learn. I'd like to get this community's input on a problem I'm having with handles that keep breaking. My favorite handle shape is on a couple of mugs I've made in the attached photo, which also shows a mug that used to have that king of handle (now broken off). The handles aren't breaking right at the connection point, but rather about a quarter-inch into the handle. This has happened to several mugs with this specific handle design (the broken one in the photo broke closest to the connection point). It takes only a mild clink with another dish from 2 inches away in the sink to break a handle off! Another thing to mention is that all of these mugs use the same clear glaze (sometimes in combination with other glazes) and it regularly has the fine, consistent crazing throughout. I've been assured by other potters at the studio that the crazing is not the problem and shouldn't make anything weaker (and that in this case it is caused by people opening the kilns while they're still too hot). Any input on why these break so easily would be greatly appreciated! I love this handle shape and it fits more comfortably in hand than any other I've tried, so I'm super bummed that they're not as strong as I think they should be. I make the handles with an extruder and handle them carefully. I trim my mugs as soon as I can safely do so, so they're still on the moist side of leather-hard when I attach the handles, which stay on very well through the rest of the process. I am wondering if bending the handles as sharply as I do (the pinched part, which I do slowly) compromises the integrity of the handle... or cutting the attachment ends at an angle with a knife perhaps stretches the clay more than a straight cut would? I'm so confused! Thanks for your help! A few other pieces of the puzzle are still missing. What type of clay are you using-what is its firing range? What temperature are you firing the glaze to? Looking at them, if you are firing in a range where your glaze is not fully vitrified they would be weak. Another thought might be the distance from the pot makes them up too much of a strain on the handle when the mug is full. Link to comment Share on other sites More sharing options...
Guest LibraryKrystal Posted January 30, 2012 Report Share Posted January 30, 2012 I am using white stoneware, firing to cone 6. These things are supplied by the studio that I rent space in, so professionals have picked out materials that (should) work together seamlessly. The glaze is mixed there. I'll be speaking with others there in more detail about this problem, but they seem confident that it's not the materials. I figured some folks in this community might have experience/knowledge in what attributes could make handles structurally weak. The handles have never broken when in use - always in the kitchen, usually during washing. And I'm actually quite careful when washing - It's been very surprising how gentle of a bump against another dish/glass will break these handles. Thanks for replying! Link to comment Share on other sites More sharing options...
Guest The Unknown Craftsman Posted January 30, 2012 Report Share Posted January 30, 2012 Those mugs look nicely done! I like the glazing on the mug on the right, I wish i had done it. I think it's a stress problem - it's hard to tell from the picture, but the handles might be too thin for the size/weight of the mug. Factor in that they are filled with liquid, which is probably hot/warm, and the added weight and heat only accelerate the cracking. It might be too much for the handle to handle. Hehheh. How long after they are in use do they break? Is in the first washing, tenth, etc. I'm guessing that it might take that long for stress cracks to develop, and eventually lead to a break when washed. Do the handles always break when you are washing them, or have they ever broken when you went to pick the mug up? I hope they don't break when filled with the hot beverage of your choice. Are any other pieces you make with this clay/glaze comination as frigile as these handles; are any attachements breaking off at all? Is anyone else using this combination, and are they having issues as well? We'll get this figured out and have you on track soon! We hope... Link to comment Share on other sites More sharing options...
Guest LibraryKrystal Posted January 30, 2012 Report Share Posted January 30, 2012 Those mugs look nicely done! I like the glazing on the mug on the right, I wish i had done it. I think it's a stress problem - it's hard to tell from the picture, but the handles might be too thin for the size/weight of the mug. Factor in that they are filled with liquid, which is probably hot/warm, and the added weight and heat only accelerate the cracking. It might be too much for the handle to handle. Hehheh. How long after they are in use do they break? Is in the first washing, tenth, etc. I'm guessing that it might take that long for stress cracks to develop, and eventually lead to a break when washed. Do the handles always break when you are washing them, or have they ever broken when you went to pick the mug up? I hope they don't break when filled with the hot beverage of your choice. Are any other pieces you make with this clay/glaze comination as frigile as these handles; are any attachements breaking off at all? Is anyone else using this combination, and are they having issues as well? We'll get this figured out and have you on track soon! We hope... Thank you! I think my strongest point is proving to be in decoration. In general, I'd say they have mostly broken somewhere around the 15th washing - say, anywhere from 2 weeks to 2 months after I bring the finished pieces home. I will try using a thicker die in the extruder next time and see if it helps. Do you know if bending the clay at a sharp angle can make it weak? Even if I have to use thicker handles, I'd still really love to be able to make them in this shape. Link to comment Share on other sites More sharing options...
bciskepottery Posted January 31, 2012 Report Share Posted January 31, 2012 Do not always assume the "professionals" have picked out materials that work together seamlessly . . . what works for them may not work for you. If your white stoneware has a firing range of ^6-10 and you are firing at ^6, then the clay is not likely to be vitrifying. That would create a weakness leading to the handles breaking. Yes, bending your extruded clay at a sharp angle will create a weak point. With your design, you could do a two piece handle that joints, via a diagonal cut at the top of the ear or "7", the two pieces. A diagonal cut would allow you to keep the shape and provide a large surface area for joining. You could work into your handle design a small ball of clay to reinforce where the handle joins the cup. A thicker coil will increase the amount of surface contact between the handle and the cup and should, hopefully, make for a stronger handle. Another alternative to consider is slip casting the handles. Link to comment Share on other sites More sharing options...
Chris Campbell Posted January 31, 2012 Report Share Posted January 31, 2012 I agree on the stress being the issue ... There is no place in your design for the handle to grab on securely to counter the force of the weight of the liquid or an accidental knock. Instead of straight joining ... by that I mean flat edge of clay to flat edge of cup ... Try using other techniques with a bit of clay for support under the joins. You could also google images of pottery mugs to see if someone else has solved the problem. Link to comment Share on other sites More sharing options...
Guest The Unknown Craftsman Posted January 31, 2012 Report Share Posted January 31, 2012 Yes, what bciskepottery said! Link to comment Share on other sites More sharing options...
Mark C. Posted January 31, 2012 Report Share Posted January 31, 2012 My 2 cents is like everyone else to some degree We do not know the clay temp and need to know if its cone 6 or something else? Add more clay to connection point-like the unknown craftsman showed in photo The crazing is a red herring-forget about it- The one thing that will help the stress of that thin handle is to extrude and form them with soft -not hard clay-when its drier (firmer) it will crack more I think the handles are getting knocked about over time (washing etc.)and -walla give it up-thicker will help If the clay is not cone 6 then all this will not help much-you need to know your materials-ask someone Also you are sure its not cone 06? as this will really make these thin handles more fragile-you should ask about that as well Hope you get to the bottom of this-let us know what you find out Mark Link to comment Share on other sites More sharing options...
Peter Posted January 31, 2012 Report Share Posted January 31, 2012 One aspect no one has touched on and considering the direction on use of materials has been sound, is the method used for curing (drying) the mugs once the handles are in place. The fact that they break away, not at the joint directly, but slightly in from that, suggests stress possibly built up as the mug dries. What is your drying method? If the handles are allowed to dry too much before installing or if the mug itself is dryed too quickly, stress may be created. Just a thought... Link to comment Share on other sites More sharing options...
dave the potter Posted January 31, 2012 Report Share Posted January 31, 2012 Using an extruder could contribute to the problem. The clay is compressed horizonally, perpendicular to the lenghth of the handle. I would try pulling them or maybe rolling coils if pulling is not in your skill set. Link to comment Share on other sites More sharing options...
JLowes Posted January 31, 2012 Report Share Posted January 31, 2012 I think Dave the Potter may be onto something. The extruded handle blank will not have the clay particles aligned as well as a rolled or pulled handle. Couple that with making a major bend in the clay an inch or so away from where the handle breaks and it may be that the handle is getting stressed in the drying more than it can take, and possibly the clay memory is at play as well. If you don't want to pull a handle, then roll a coil, and slap it down on the table to flatten it. That will also compress the clay some too. Then as someone else suggested, make the major direction change by cutting the coil into two pieces and joining it well by scoring and slipping. The area where you are getting the break is also one where it is easy to overwork the clay trying to get the angle just right; especially so if the clay is too moist. Make a few cups, and put 4, 6 or 8 handles on them to test different designs for the shape you want, extruded and coil. That way you will get a lot of practice without making a bunch of cups. John Link to comment Share on other sites More sharing options...
Ben Posted January 31, 2012 Report Share Posted January 31, 2012 "Another thing to mention is that all of these mugs use the same clear glaze (sometimes in combination with other glazes) and it regularly has the fine, consistent crazing throughout. I've been assured by other potters at the studio that the crazing is not the problem and shouldn't make anything weaker (and that in this case it is caused by people opening the kilns while they're still too hot)." I will echo that it is important to know what temp the clay is rated to and what temp it is fired to. This will give some clue as to how vitrified the pot is. Along those line of thinking be aware that there is no such thing as cone 6~10 clay. If it is properly fired at cone 6 it will be well overfired at cone 10 and vice versa. I have to take exception to assertions by the other potters in your studio that the crazing you see is benign and that it is caused by opening the kiln too soon. Crazing happens because of a mismatch in the expansion/contraction rate of the clay and glaze. In this case the glaze needs to shrink more as it cools than the clay does and since the glaze is a glass, and cooled beyond the point where it it molten, it breaks. Crazing can be a great tool for letting you know just how big a mismatch there is between your clay and your glaze. The more widespread and smaller the crazing, the greater the mismatch between the glaze and body. Widespread, fine crazing is not caused by opening the kiln too soon. It can be made to show itself quicker by a faster cooling such as opening a kiln but the mismatch in expansion that causes the crazing will be there either way and eventually, maybe a week or month later, the crazing will show. The crazing you see is a clear indication that you have a substantial mismatch between the clay and glaze expansion coefficients. This mismatch cannot be good for your pots and may be contributing in part to your problems with the handles. It certainly isn't adding any strength. I think that you should also examine the way you make these handles as others have suggested. I especially like the idea bciskepottery proposes. That cut and slipped joint would be strong. Would you mind posting the clay and glaze formulas or brand names? That would help a lot. Ben Link to comment Share on other sites More sharing options...
ayjay Posted January 31, 2012 Report Share Posted January 31, 2012 I used to make mug handles which were not exactly extruded but made using a home-made loop tool pulled through a block of clay - these handles have proved to be less long lasting than the pulled handles I now make. Give pulling a try. Link to comment Share on other sites More sharing options...
teardrop Posted January 31, 2012 Report Share Posted January 31, 2012 We viewed this video on making handles during a class lecture last semester and I found it to be highly informative and very helpful. Link to comment Share on other sites More sharing options...
Guest LibraryKrystal Posted January 31, 2012 Report Share Posted January 31, 2012 Thank you so much, everyone! After some digging, I found that the stoneware is Laguna's 510, and on their site it's labeled as cone 8 - 10... So the fact that the studio fires everything at cone 6 could certainly be a problem! The glaze is mixed in the studio and I don't have access to the recipes, so I have no idea what kind of shrinkage percent it is prone to. I have also found that the pottery world is quite divided on the issue of crazing and whether or not it weakens pots. Does anyone know of any studies done to try to determine a verdict on this? Some detail on my process: As for making the handles - I make them with fresh, soft clay in a wall-mounted extruder in 2-foot-long pieces laid carefully on a board. I cut a piece off using a knife and then bend it slowly while pressing gently (the spot where it is pinched) to keep the clay from getting any of those soft cracks it can get when being bent at such an angle. Then I give that spot another pinch with my fingers lined up so I get a nice even pinch shape there. After that I simply find a good fit on my mug and slice the ends of the handle at an angle to have a larger connection area where it joins to the mug. I score the connection points (both mug and end of handle) and add a touch of water or slip before pressing the handle ends into place. I tend to trim my mugs as soon as I safely can so that they still have plenty of moisture in them when I attach handles ( - I rarely have any issue with handle attachments separating or cracking away from the mug when I use this timing). I try to let my mugs dry as slowly as possible. As I'm attaching the handles on a batch of mugs, I set each finished one on a thick towel placed around my wheel, so the mugs can set on their sides with the handles going straight up to avoid upward or downward stress/sagging on the handles. When I've got them all attached, I set them upside-down on a board and cover it securely with plastic sheeting. They will stay this way (for 7 - 12 days or so) until nearly bone dry and ready for decoration and bisque firing. I know I have a lot to learn and I really appreciate all the information and suggestions! I will definitely try using a thicker hole in the extruder, as well as pulling handles manually, since I can't change what clay the studio purchases for members to use (although I will certainly be bringing up these issues with the studio's managers). Link to comment Share on other sites More sharing options...
Mark C. Posted February 1, 2012 Report Share Posted February 1, 2012 I am totally going with the clay body being under fired. My experience is extruded handles are stronger than pulled handles. Test this by breaking each type-the extruded ones are even thickness and compressed more than any pulled handle. Yes pulled handles do look better but for me not as strong. I only cover my porcelain mugs 1 night with plastic-so you are really drying very slow my only suggestion is use slip not water at the connection points Happy potting mark Link to comment Share on other sites More sharing options...
teardrop Posted February 1, 2012 Report Share Posted February 1, 2012 Score one for the noobs! Wow.... somebody is failing you big time in that studio (IMO). Good thing you are on it! Are any of the other folks working with you in studio experiencing breakage of their peices/etc. as well?? good luck! Link to comment Share on other sites More sharing options...
MadMudder Posted February 1, 2012 Report Share Posted February 1, 2012 You need more BEEF in the handle. Even though the skinny little things look cool they are non functional, find a way to thicken them up. The place where they have broken is so tiny.... YMMV B Link to comment Share on other sites More sharing options...
bciskepottery Posted February 1, 2012 Report Share Posted February 1, 2012 Try to find a true ^6 clay body. The Laguna 510 is definitely not maturing at ^6 and is the likely source of the handle failure. Because the clay is not vitrifying, it is likely absorbing water during washings, etc. and could explain why the handles are breaking after several washings. I would suspect the crazing of the clear glaze is not the result of people opening the kiln too soon, but the poor fit between the glaze and 510 clay body. At ^6, the 510 clay body will not complete shrinking because it is not reaching vitrifying temperatures; it will remain porous, absorbent, and somewhat weak so that over time it eventually fails at a stress point -- the handle. Crazing in a liner glaze for functional ware is something you should be concerned about, in my opinion. Each repeated episode of hot coffee/tea/chocolate and washing in hot water (by hand or by dishwasher) exacerbates the glaze fit (or non fit) and crazing. Crazing that stays below the surface of the glaze is one thing, but crazing you can feel on the surface is another. Chances are the glaze will fit better on a true ^6 clay body. Been there, done that. Link to comment Share on other sites More sharing options...
LawPots Posted February 1, 2012 Report Share Posted February 1, 2012 Michael Cardew said in Pioneer Pottery that: "vitreous ware with a non-crazing glaze may be three or four times stronger than the ware which is crazed." Do not be too quick to discount the possibility that the crazing is effecting the strength of you handles. Link to comment Share on other sites More sharing options...
Ben Posted February 1, 2012 Report Share Posted February 1, 2012 "After some digging, I found that the stoneware is Laguna's 510, and on their site it's labeled as cone 8 - 10... So the fact that the studio fires everything at cone 6 could certainly be a problem! " Yes, it most certainly is the problem. Are you paying to use this studio? If so then it would be the same as paying for tires for your car only to have the store install the wrong size and them expecting you to be happy with that. No way you should accept that. " The glaze is mixed in the studio and I don't have access to the recipes, so I have no idea what kind of shrinkage percent it is prone to." It isn't the shrinkage we are worried about but the expansion and contraction. Shrinkage in clay is due to drying. Expansion is due to heating or cooling. Most matter will expand and contract due to heating and cooling and this is independent from shrinkage. It is frustrating to not have access to the glaze data. Have you tried asking for the recipe? If you get it we could help you find a clay that might be better suited to that glaze. "I have also found that the pottery world is quite divided on the issue of crazing and whether or not it weakens pots. Does anyone know of any studies done to try to determine a verdict on this?" The pottery world may be divided but the ceramic science world will not be. Strength due to glaze fit is emperical and easy to test. Test bars are glazed and fired then broken on a machine that records how much force is needed to break the bar. I'd also be interested to see any published data on this subject. Does anyone know of any? " since I can't change what clay the studio purchases for members to use (although I will certainly be bringing up these issues with the studio's managers)." That may well be the way to affect a change in the studios procedures. If your goal is to make quality, functional ware, they are not providing you with the right materials given their glaze and firing procedure. At cone six you will need a different clay. A properly matured clay should cure your fired strength issues for the most part. It may or may not fit with the glaze. If you have the option of having this clay fired to a higher temp that too will make the fired clay stronger but you will likely need a different glaze. Talk to the studio. The easiest fix would be to change clays. Link to comment Share on other sites More sharing options...
Ben Posted February 1, 2012 Report Share Posted February 1, 2012 Found some data on fired strength vs glaze fit. http://books.google....TRENGTH&f=false from the Journal of the American Ceramic Society, Vol 5 and this from Ceramic arts daily http://ceramicartsda...file-glaze-fit/ and this from the digitalfire website http://digitalfire.c...rength_202.html Link to comment Share on other sites More sharing options...
Guest LibraryKrystal Posted February 1, 2012 Report Share Posted February 1, 2012 Thank you, Ben! I will see if I can get my hands on the glaze recipe. Since I am in a very small city, I don't have other studios to consider... and yes, I'm paying to use the studio and for use of materials. I've come up with one more big question, and it's definitely a beginner's question, but here it goes: If I can get them to bisque fire the Laguna 510 at cone 10, is it OK to then do the glaze firing at cone 6? (All the glazes seem to be truly cone 6 - I've seen underfires and overfires and they are clearly that.) Will the clay have been completely vitrified from the bisque firing and stay that way through the lower-temp glaze firing? One other thing - I took a mug (that turned out ugly) with that same handle type and banged it around in the sink, hitting the handle with the edge of another old cup. It didn't break. So now, I'm letting the mug sit in a bowl of water for a while (with the foot of exposed clay submerged) to see if that makes it break easily. Link to comment Share on other sites More sharing options...
Pres Posted February 1, 2012 Report Share Posted February 1, 2012 Thank you so much, everyone! After some digging, I found that the stoneware is Laguna's 510, and on their site it's labeled as cone 8 - 10... So the fact that the studio fires everything at cone 6 could certainly be a problem! The glaze is mixed in the studio and I don't have access to the recipes, so I have no idea what kind of shrinkage percent it is prone to. I have also found that the pottery world is quite divided on the issue of crazing and whether or not it weakens pots. Does anyone know of any studies done to try to determine a verdict on this? Some detail on my process: As for making the handles - I make them with fresh, soft clay in a wall-mounted extruder in 2-foot-long pieces laid carefully on a board. I cut a piece off using a knife and then bend it slowly while pressing gently (the spot where it is pinched) to keep the clay from getting any of those soft cracks it can get when being bent at such an angle. Then I give that spot another pinch with my fingers lined up so I get a nice even pinch shape there. After that I simply find a good fit on my mug and slice the ends of the handle at an angle to have a larger connection area where it joins to the mug. I score the connection points (both mug and end of handle) and add a touch of water or slip before pressing the handle ends into place. I tend to trim my mugs as soon as I safely can so that they still have plenty of moisture in them when I attach handles ( - I rarely have any issue with handle attachments separating or cracking away from the mug when I use this timing). I try to let my mugs dry as slowly as possible. As I'm attaching the handles on a batch of mugs, I set each finished one on a thick towel placed around my wheel, so the mugs can set on their sides with the handles going straight up to avoid upward or downward stress/sagging on the handles. When I've got them all attached, I set them upside-down on a board and cover it securely with plastic sheeting. They will stay this way (for 7 - 12 days or so) until nearly bone dry and ready for decoration and bisque firing. I know I have a lot to learn and I really appreciate all the information and suggestions! I will definitely try using a thicker hole in the extruder, as well as pulling handles manually, since I can't change what clay the studio purchases for members to use (although I will certainly be bringing up these issues with the studio's managers). It is good to hear that our original premise(under fired clay) holds up. It is a shame that all too many times studios do not keep up on what is going on. Often a change in management, or knowledgeable personnel, a change in firing as changing from a gas kiln and higher temps will have people trying to use the same clay body because they like it. All too often these factors come to play with the overall performance of the studio. I would talk to someone about the clay body being ^8-10, and suggest that they use a body in a closer range to ^6. After all it doesn't hurt to ask-what are they going to do? Say yes or no. If this gets you no help, I would consider buying my own clay and working in the studio with that. Make certain they see the box, and that it is a true ^6 clay to assuage their fears that you are using a lower fire clay that will melt in the firing. Some shops will not allow any outside clay in the studio, but in this case. . . I think you have a valid argument. Link to comment Share on other sites More sharing options...
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