Help with slumping molds: finished pieces rock

[Shelley Lewis]

Most of my slumped pieces rock slightly. It's happening with bowl and plate molds. Is there something I can do to prevent this? Maybe I just have poor quality molds? My kiln is level, and I make sure the molds are level when slumping. Unfortunately I don't own a grinder, so I'm not sure how to go about fixing the problem. Thanks for any suggestions on fixing the molds and/or finished pieces!

[Brad Walker]

What kind of molds are you using? Most often, if you have this kind of problem, it's from using bisque ware rather than molds made for slumping glass.

One no-equipment-needed fix is to use bumpons to even out the bottom so it sits well.

[Shelley Lewis]

Hmmm, I guess I'm not sure what kind of molds I have. I have bought all of them from slumpys or delphi. I just assumed they were all designed for glass slumping, but maybe I'm wrong???

[Brad Walker]

Hmmm, I guess I'm not sure what kind of molds I have. I have bought all of them from slumpys or delphi. I just assumed they were all designed for glass slumping, but maybe I'm wrong???

Just because someone sells a mold for slumping doesn't mean it was designed for slumping.

Look at the bottom of the mold. Is it nicely finished, like a plate or bowl, or is it rough? In a mold made for slumping, a lot of attention is paid to the top surface of the mold, where the glass will shape around. In a mold made for use as pottery, a lot of attention is paid to the bottom of the mold, which doesn't matter to a glass artist. And these types of molds don't always have a smooth, even surface, on top.

It's also possible that you have a mold that is curved without a flat area for glass to sit on the table after slumping. That "ball surface" mold isn't really designed for plates and bowls without some additional alteration to the glass.

[JestersBaubles]

I have found that even with high-quality, Bullseye molds, you will occasionally have one that just doesn't sit right.

That being said, often a "rocky-bowl" is due to the fact that you didn't quite slump long enough for the glass to conform completely to the mold -- any shiny spots on the bottom? (that would indicate that the glass didn't actually touch the bottom of the mold)

I have a friend who uses the bump-on method. My problem with this is that you (and more importantly, your customer) have to be careful when washing the piece and eventually those bump-ons will bump off . You could also fuse small glass cabs and then glue them on w/E6000, but in my experience, they do require some grinding to level out. There are also now "feet"molds that are designed to create glass pieces (they don't look like feet -- some of them are leaves ) that can be adhered to your piece after slumping.

Lastly, a way to cold work them without equipment is to put loose grit on a piece of flat glass (float works), add some water, and run the piece in a figure eight back and forth. Bullseye has a video in their education section (accessed via subscription, but I think their info is worth the $39/year) that is all about working with loose grit, and Paul Tarlow published a book, "Coldworking without machines". http://coldworkingglass.com/

Dana W.

[The Hobbyist]

I have used this method:

Put the offending bowl in the kiln and note where to touch it so that it will rock.

Then heat the kiln until it approaches the slumping temp range. As it gets close reach in and touch the bowl to make it rock. Eventually as the temp/time rises there will come a point where it will not rock. At this point crash cool the kiln to stop the slump and proceed to anneal.

Be sure to observe all the necessary cautions regarding ramp up, anneal processes and playing around in a working kiln.

The bowl does not lose its shape because it has not been in the slumping temp range long enough. All the weight is concentrated at the spot you want to flatten and when it is softened it will deform first. The crash cool will prevent any further changes to the bowl.

Works for me.

Jim "The Hobbyist"

[Shelley Lewis]

Thanks to everyone for your advice! I'm going to try all the suggestions

[Laurie Spray]

This is exactly why we make our mold " Bottomless"!!

[DonMcClennen]

Set a piece of 1/8" fibre half the size of the base on the centre of the mold. Piece will slump over causing slight recess in underside with ring around it . This will stop rock.

[Deive Butvila]

Set a piece of 1/8" fibre half the size of the base on the centre of the mold. Piece will slump over causing slight recess in underside with ring around it . This will stop rock.

This is really a neat trick. I was very disappointed with the results I got using an expensive ($50) mold that produced a rocking bottom. Trolled the forum and found this bit of advice. Bought some 1/8" fiber and it saved the piece and mold from going into the trash.

Many thanks