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Trimming Dilemma


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I am an experienced potter but I’m having a consistent and nagging problem when I trim my pots.  When I throw, I work on a very flat Masonite bat attached with bat pins to my wheel head. My pieces are completely centered and always feel level and completely round. When I trim, the levelness goes totally wonky. This has been happening for the past couple months. I feel like the pot has a large lump on one side and I can’t get an even foot ring or maintain smooth trimming. I usually trim on a foam bat. I have used new, well wedged clay without air pockets. I have leveled my wheel. What are some other things I could consider when troubleshooting this problem?

9828B53B-5463-40A3-AA06-4B40BED46ED8.jpeg

Edited by Gayle Clay
Mistake
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  • Gayle Clay changed the title to Trimming Dilemma

Hi Gayle!

A few thoughts:

  Pieces that come off the wheel round and even may not stay round and proportioned, for handling, uneven drying, uneven thickness... particularly open forms, and no doubt, large forms, as Liam points out. If there's any air movement, any sunshine, turn those drying pots periodically. Hadn't though of flipping 'em, good idea. I place ware on plaster as they're getting close to trimmable, which draws some moisture out the bottom. 

  Centering up for trimming, pick what part to center to. Typically, I'm keying in on just above where I'll stop trimming, not the rim, nor the edge of the base.

  Tools want to follow the contours of the work. To force round and level, brace the tool and proceed patiently to get that foot ring round and level.

I start out with removing most o' the excess with a loop tool where the wall meets the base, then within the foot ring. From there, I use the needle tool - which doesn't follow the contour of the clay - to cut away and form the bottom and side edges of the foot ring, round, level. Next, my bladed tool to finish up, then burnish, and mark it up with chattering and grooves.

Nice profile on that bowl!

Edited by Hulk
counter->contour
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Are you sure your foam trimming bat is ok?  I remember being at a workshop where  the potters beside me  was trimming on a foam bat that was completely wonky.  She didn't believe it when the instructor told her that that was her problem.  Get your foam bat spinning on the wheel.  Possibly put a piece on it.  Start the wheel turning then stand away from the wheel and you should be able to  see if the bat  is behaving as it should. Lin

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When I trim, after centering, I lock in my elbows etc and use a broad, long chef knife to level the base, holding it at right angle to the pot base. The knife length covers the entire base

After that I define my outside of foot.

Maybe your pot is not of uniform dryness and so the tool pressure does not have same effect  all the way around your pot.

Maybe??

Be firm!!!!

Can you stabilise your tool so it is only trimming the hump?

Or recentre pot to see if makes a difference?

Which part of the pot are you centering from initialky?

Are you just using the rings on your foam?

Edited by Babs
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Yes. Definitely happening on larger, wide bottom bowls. I had been struggling with winter drying issues and cracking that I remedied by drying pieces on their rims. I realize now that my heated studio tends to be dry and it does get a blast of sun all morning. I am going to explore the process of turning the pieces more often and will actively pay tons of attention to the drying process. Will get some plaster bats, and sharpen my trimming tools. Everyone’s suggestions were wonderful. This problem has been keeping me up nights, so THANK YOU so much!.

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Another thing that I found when making larger bowls is that if you trim some of the excess with a wooden knife at the end of the throwing, you can push it off centre a bit. It’ll mess with your ability to trim. For larger bowls, cut the excess clay away with a pin tool if you need to, or just tidy it up enough to get the wire under it and leave it for the loop tool later.

Another tip is that the larger the pot, the easier it is for the foot to be subtly out of alignment with the rim, and for both of those points to be out of alignment with the transition point from foot to wall. It’s not always visible unless the pot is actually spinning. If you’re re-centring to the rim or the foot and that transition point  is where you need to remove the most clay from, you’ll go through the side. You need to re-centre that transition point, not the rim or the foot.

Edit: after having a much closer look at your image, I think that last tip is what’s happening with you. You’ve got your pot rim lined up perfectly with the marks on your bat. You need to train your eye to centre that shoulder.

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ran into this problem years ago.  someone watched and pointed out what i could not see.   i released pressure on one side while throwing, leaving a thick place in the wall.   it does not go away while drying or trimming.

you might find that a mirror placed so you can see what the profile is as you throw will help enormously.    and will save your back and joints from years of stretching to see the profile when you cannot clearly see it.

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I have this problem on a regular basis in my drafty studio.  It’s because one side of your pot is drying faster than the other side. The softer side is easier to trim than the harder side, so the trimming tool makes faster progress on that side. 

I can even things out a lot by draping fabric over my thrown pieces until they are leather hard. It creates a mini-environment that is evenly humid under the fabric, and guards against drafts. I still sometimes get unevenly dry pots to trim, so when I see it happening, I brace myself into a firm hunch and hold  my trimming tool with two fists. This makes sure I am holding the trimming tool still, and not letting it rest on the pot. 

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When I'm looking at your photo I'm also seeing the curve different on opposite sides of the bowl, if you were to cut this open I believe you would see the side on the left is thinner walled than the right side of the bowl. I think this is also from having the clay firmer on one side versus the other. I've run into the uneven footring depth a few times with larger bowls when I try and dry them too quickly in my drafty workroom. To trim I find using a sharp and very small wire loop cutting tool to really cut into the clay can even things up. 

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Re getting the foot level, instead of levelling the entire base it's easier to trim just the footring level (if that's even an issue). After doing the vertical edges make 2 cut angles then just use your loop tool held horizontally and cut the top off. 

1677839249_ScreenShot2021-03-18at8_42_37AM.png.8a8868207cf7c6f6df10ed4739cb8579.png

Edited by Min
grammar
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Thanks for even more terrific ideas to consider, which I will. Plan to spend the afternoon making bowls for testing out suggested centering, trimming and drying methods.  I’ve also realized that I did not have this problem during warmer months  when my studio is open to the outside and the heater is off.

Question for Mea: when you drape your pieces with fabric, what type of ware board or surface are they on? Are they turned over on their rims?

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39 minutes ago, Gayle Clay said:

Question for Mea: when you drape your pieces with fabric, what type of ware board or surface are they on? Are they turned over on their rims?

My pots are still on their throwing bats through this stage, right side up. Any pot that is 6 lbs or more will probably be on a Hydrobat, which helps with even drying. And this problem is definitely worse in the dry winter climate too, compared to my humid summer climate. 

Edited by GEP
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In your photo you can see that the foam is compressed more on the right side as that side has trim marks way down lower than other side on left where at top you have an issue -ditch the foam and trim on a wheel head or bat with wet clay balls until you learn the foam pad deal. 

Notice the uneven trim marks on bowl sides down where you stopped trimming-that line should be straight

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If issues with the foam are not there, I might give another suggestion to try. As others have said, uneven drying can cause some shrinkage/shape problems. I have had this happen before to me on some of my large patens(communion plates) that I throw. My solution has been to use a pointed or small diameter trimming tool held in both hands braced on my legs. I put a series of concentric rings about 1/4 " apart. I usually trim to the same depth down. I even do this on the curved areas.  Then I use a very sharp flat tool to trim the hole area to the depth of the first cuts. I then repeat until I have successfully trimmed the piece. With plates I have to be careful with the cantilever where it is the rim supported by the clay at the base. With bowls you do not have that problem. Check thickness often, and you should do fine with this technique.

 

best,

Pres 

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  • 2 weeks later...

I had this same issue - I finally realized my foam bat was cut unevenly (I was using one from Diamond Core tools). I then bought two foam bats (both the thicker and thinner ones they offer) from StudioPro Bats and they have been great. I notice you have a StudioPro foam bat - I'd make sure you don't stack other bats on top of it making it uneven, maybe?

I still use that uneven bat as a soft yet sturdy surface while I carve into mugs, bowls, etc

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I would slice some in half to see what is going on and when. Before coming off the bat, right off the bat, before trimming and after. I'm with Mark on the foam trimming bat being the culprit but maybe its just the angle of the picture. Seems like everyone covered lots of possibilities though so if you can narrow it down for certain when it is happening one of the causes might come into focus. Good luck! 

edit: I only use foam bats for things that are really only getting a little light clean up like cups and mugs. Anything that needs a foot trimmed in happens on hard bat.

Edited by Stephen
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When you say that your pieces feel level when you are throwing have you tested this with a needle tool at the rim?  Sometimes  the pot can feel level but when a needle tool is held at the rim the unevenness can be detected.  Lin

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