Bottle Slumping Tips #1: Preparation

Since our first Bottle Slumping Tip, we've gotten a lot of questions about slumping bottles. Many of the questions come from people who purchased our Trio glass slumping kiln. Others are related to the bottle molds we sell. Therefore these questions may seem specific to the Trio and bottle slumping, but they actually apply well to many glass slumping situations. We've categorized these are firing preparation questions.

  • Yes, absolutely. You need to put some type of "release" product on any surface in the kiln that glass may touch, including the mold surface. You can use kiln wash, or glass separator, or any of a number of different release products available, such as Primo Primer.

  • Personally, I use kiln wash a lot, because I happen to have it hanging around my ceramic studio. For practical purposes, it is the same as glass separator, except glass separator is more finely ground and arguably leaves a smoother surface. Glass separator also holds better on some surfaces like bead mandrels. (If you need some advice on how to mix kiln wash, see Tip 96.) Therefore, use glass separator if you have it. If you only have kiln wash, that's fine too.

    I put kiln wash on the kiln shelf (top surface). I put glass separator or kiln wash on the mold surface (where the bottle will touch, not the entire thing). I put on 2 or 3 fairly heavy coats. If you're slumping onto a more intricately surfaced mold, like one of these texture molds, heavy coats of kiln wash can diminish the impact of the texture. In those cases you'll want to consider using an alternate product like Primo Primer, to get as much texture as possible into the glass.

    If you're fusing flat on a kiln shelf, then shelf paper is nice to use because you won't risk seeing any brush strokes from applying the kiln wash.

  • You want to re apply whenever part of the mold surface has become exposed, or if the surface has become un-level (because some of it has come off). As long as it's still there and looks compete, you're good to go. If I'm firing regular bottles onto a smooth mold, I can typically go 5 or more firings before I need to do a touchup, or start fresh with new kiln wash. When in doubt, apply new. It's no fun when your bottle sticks to your mold, because then your mold is generally a lost cause.

    At the end of every firing, I recommend you inspect the mold and see how the kiln wash looks. If it needs touching up, or a new coat, do it then, so it can be drying while you get ready for your next firing.

  • I mentioned 2 or 3 heavy coats of kiln wash in a question above. I do that because I like slumping bottles with permanent lettering, like many vodka and Corona beer bottles. The lettering on the back side of the bottle really likes to stick to the kiln wash, and will pull it all up, right down to the mold surface. So, in an abundance of caution, I put on a lot of kiln wash. In these cases, I have to re-apply wash after every firing. Sometimes I'll just fill in the pulled off sections, if I'm not being too fussy about the smooth/level-ness of the mold surface, If you want it really smooth, you'll have to remove it all and start fresh. Alternately, you can try to touch it up and work it level again before your next firing. (I have not found a good way of doing this yet, or I'd tell you how. I tend to rub the kiln wash surface a bit with the tip of my finger, and that helps smooth it out.)

  • I take the bottles to my laundry tub. While running water over it, I scrub the kiln wash away with a nylon bristle brush. It comes off pretty quickly, with no damage to the lettering.

  • In our first tip, we discuss soaking the bottles to get the labels off. That works, but this seems to be simpler (or at least an alternative you can try). Most labels are attached with heat sensitive glue. If you heat the label with a stripper/heat gun for a minute or so, you can normally peel the label right off. Hairdryers do not get the bottle hot enough (at least not in the amount of time I've been willing to wait around), so you have to use a heat gun. You can pick up a cheap heat gun for $25 or less that works fine for this. The bottle will be hot around the label, so you have to carefully flick the edge up with your fingernail, or some tool, until you can get your fingers tips on both sides of the label and pull it off.

    There is normally some residual glue left on the bottle. Sometimes sticking part of the removed label back down with your thumb and pulling it quickly up will get some of the residual glue off. Alternately, Goo Gone also works really well on the leftover adhesive. Put the Goo Gone on a paper towel and wipe the adhesive off.

    Note: This removal technique will get the label off in one piece, in nearly perfect condition. You can then re-use the label to re-decorate the bottle once it's slumped. I've seen some nice examples where the label has been re-glued to the underside of the slumped bottled, so that it shows through the bottle and creates a very nice effect.

  • The bottle needs to be super clean. Anything on the surface of the bottle (leftover label glue, fingerprints, etc) can cause the slumped glass to have surface defects, the main concern being devitrification, which clouds the glass by causing the glass to crystallize in those areas (read more about devitrification at Wikipedia).

    I've had good success by using the following method: I wash the de-labeled bottle in hot soapy water, inside and out. I rinse it with fresh water and dry it with a clean towel. I minimize further handling until I'm ready to load the kiln. Right before loading, I wipe down the bottle with isopropyl alcohol (IPA. The cheap 70% stuff seems to work fine.) My technique is to hold the bottle with a paper towel in one hand, while I wipe it with another paper towel (with IPA on it) in my other hand. I never let my bare hands touch the glass. I then set the clean bottle immediately (into the mold) in the kiln.

    If you don't want to be that careful, or if you want an additional level of protection against devitrification, you can use a devitrification spray, like Fuse Master Super Spray. It works very well. It coats the bottle with an overglaze that inhibits the devitrification process.

  • Good question because people sometimes forget about this. Bottles can roll. If your kiln bottom is not perfectly level, you'll notice this immediately when you try to place the bottle in the kiln.

    There are two concerns. First, if you have a permanent label on a bottle (like a Corona beer bottle), you'll want the label to remain straight up, so it looks correct when slumped. If the bottle rolls, the slump will not look very good. The second problem is actually much worse. The bottle may roll all the way to the wall of your kiln and melt there, doing damage to your kiln.

    First, try to get your kiln level. Next, block the rolling motion of the bottle by putting a little bit of clear glass frit (crushed glass pieces) under the edge of both sides of the bottle (similar to how someone would put wheel chocks on both sides of a tire, to prevent a car from rolling). Once the bottle begins to slump (at about 1300 F), it won't be able to roll anymore and the clear frit will melt into the bottle. Even if you are slumping a colored bottle, you will not notice a tiny bit of clear glass on the bottom side. The frit is such a small amount of glass that any difference in COE (rate of thermal expansion of the glass) between the frit and the bottle is not a problem either. I put the frit near the bottom end of the bottle, where it will be additionally hidden by the slumped bottle bottom.

    As a second precaution, I often lie a kiln post down along the edge of the kiln shelf, parallel to the bottle. Should the bottle somehow overcome the frit (or if for some reason I do not want to use frit), the bottle can not roll any further than the kiln post, and the kiln wall is spared any damage. Worst case, I have to throw away a kiln post, which is a small sacrifice to protect the kiln.

See our other Bottle Slumping Tips and browse our massive selection of kilns:

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