Kiln Stilt research and recommendation

[Shirley]

Recently there’s been a bit of discussion back in FA about stilts - fueled by why did the commercial stilt under @meanbaby big bowl break. As requested, I did some further research and have what I believe is a pretty reasonable explanation. Also, I have recommendations for the next time a piece needs to be elevated and stilting is considered.

There are 2 primary types of commercial stilts available. The metal pin set in a clay body type and the all ceramic type. (There are some other stilt like objects sold such as spurs, but are not a major offering). I know we have the pin type. I haven’t looked to see if we have the all ceramic type. Both come in a variety of sizes and shapes. No matter which type you use, both have several considerations you need to keep in mind:

1. Use an appropriate size stilt for the piece (sizing to the base is a good way to start). If you have a big piece use a big stilt, multiple smaller stilts might not handle the load as well.
2. Make sure there are as many points of contact as possible for the larger pieces (load spreading) to avoid breaking the stilt.
3. Clay warps more in higher temperatures. Just something to keep in mind when deciding stilt placement.

Specifics to the types-
Metal pins set in ceramic base

1. These are really designed for bisque/low fire temperatures.
2. While the materials they are made of can go higher - such as cone six or even cone 10 - they really should only be used with smaller/lighter pieces at these temps if at all because -
3. Firing the stilt causes the materials it is made from to degrade. The higher you fire it, the faster the degradation.
4. Firing at 6 or higher causes the pins to soften and bend making these types of stilts a poor choice for mid to high temp firings especially if loading a large piece. Over time they can warp or bend Or
5. The sharp metal points can become embedded/causing scarring where it touches the clay. Temperature, clay body and size, will influence which will happen.
   My opinion (& general pottery concensus) - Pin stilts should be used for bisque firing with the very rare mid firing of small piece only, if at all. While they can survive the firing temp especially with lighter pieces, each firing degrades the material and risks a failure. There may or may not be any visual indicators. Depending on how the piece is being held up or how the stilt fails, the result could be minor to completely catastrophic (piece falls sideways taking out whatever is next to it).

I’m fairly certain this is what happened with the stilt holding the large bowl. The stilt was fired at 6 before an unknown number of times, weakening the clay body. A heavy load was placed on it spread to only a small number of pins. The glaze firing plus load pushing down on so few pin caused too much stress at the point on the stilt arm and it broke. There’s also no telling if the stilt had an existing flaw or weakness exacerbated by previous firing or the overload plus glaze temperature.

All ceramic stilts

1. Designed for mid to high fire stilting
2. No metal failure point
3. Can be combined with Kiln Wash for easier removal
4. Points are known to break easily from some brands
5. Stilt molds exist
6. Points can become stuck to the piece (see #4 for reducing this)
   My opinion - between the two types of commercial stilts, this is the better one to go with, especially if it’s a larger piece in our glaze firing. It’s definitely the safer one to go with. Do we have any all ceramic stilts?

My actual recommendation - I have 2 solutions to recommend. Which one is best is highly depend on the piece, glaze, glaze application and why you’re stilting a piece (more airflow under, firing sideways, nested firing, runny glaze, deliberately not dry footing, etc).

1. Pedestal glaze trays - A stilt with a built in glaze tray? Brilliant! These can be thrown on the wheel in whatever clay you want in various sizes. Bisque fire and keep them until needed or make one to go with a particular piece. Fire them in bisque together. You can even hand build them. If anyone needs to see examples or would like a lesson/demo on making these please ask. Same for standard glaze trays.

2. Wadding plus glaze tray. Good wadding allows flexibly in use (shape, placement, and amount used), fires well into cone 12+, is fast to apply, easy to use and comes off the piece clean. I’ll get with @cmcooper0 and we will see about getting hold of a recipe we like and mixing it up. Wadding a piece is my go to method for stilting.

[meanbaby]

Good info for newby’s! My stilts I special ordered as a high fire stilts, they are really old, and used multiple times, fired at cone 6, at more than one studio with no issues. Some of them are not in the best of shape due to ware and missing points. Could have been something as simple as hot spot in the kiln .It was just its time to go.

[Shirley]

Ah, Gatcha! I wasn’t told you had supplied your own stilts. I’ll be honest, I didn’t ask too many questions at the time, I was in the middle of teaching class and was trying not to split my attention too much from them. Before then, the broken stilts had been a topic of conversation back in FA and don’t remember hearing it said then either. It may have been and I just missed it. I assumed they were DMS supplied ones and I was under the impression those are the standard ones you can get from amaco.

Were yours Roselli stilts? Those I know are designed to fire to cone 10 and I’m curious about them. I was wondering how well they hold up in the higher Temps and for how long/how many firings before they become too risky to use. Reviews seem mixed and they still seem prone to fail as expected in the manner pin stilts do - at the pins. I’d rather get a review from someone I trust on them!

I absolutely agree it could be as simple as a hot spot in the kiln that killed the stlt. I’d love to find out what the kiln is actually doing.

[dryad2b]

I’m suspecting that @meanbaby’s weren’t meant to go to Cone 10, because I stuck a glaze tray under the setup just in case of big drips, and the stilt had significantly stuck to the glaze tray. The pot didn’t run or drip – but the stilt stuck to the glaze tray, in addition to having the metal spikes completely bend flat under the pot.

I am currently very wary of the stilts DMS owns. When I’ve used two or three of them, slightly larger pots have had the points fail. They are only good for very small things at this point. They’ve all clearly been used, and they predate me, so I’m not sure how many times they’ve been used. We’ve got some single metal points (think caltrop) which say they’re good for Cone 01. So – I use those once and toss them. Thus far they don’t fail in one time use, and I haven’t given them a chance to fail by using them a second time. I suppose we could conduct tests, using a glaze tray and maybe just refiring the same test piece over and over until the points failed. Or not.

[Shirley]

The ones out there targeted specifically for cone 6 firings are designed to go to and survive multiple firings at 10. I’m not aware of any that are marketed to cone 6 that also can’t survive up to 10. If you know of any brands and where to get them, please let us know. It will be great info to have for the folks interested in stilting glazed pieces. It’s nothing for the manufacturer to switch out the clay body used in the base so you can slap the extra number range on the packaging. In pin stilts, it’s generally not the base but the pins that fail under a heavy load - but as I’ve said, that’s in general for pin stilts. There’s ways to minimize this.

I’ll wait on her to confirm either the brand or firing range as stated by the manufacturer. I’m super curious about the stilts she has. If they are the ones I think they are (Roselli - now owned by The Ceramic Shop, but you can get them from any of the online ceramic dealers like big ceramics or clay king), I have questions for her about them and then I’m probably going to acquire some for myself.

I’d completely forgotten about the glaze tray being stuck onto the stilt. I’ll admit it: @meanbaby’s piece is awesome! I’ve only ever spared a glance at the broken stilt before having my attention completely sidetracked by it. Who wants to look at a stilt when you have that bowl to look at instead!?!

Since the kiln is programmed to go to 6 and her stilts have been fired to 6 without issue at multiple other studios, the fused glaze tray in the absence of any glaze is what concerns me. Ultimately it may be the actual cause of the stilt break.

One possibility - The kiln overfired, causing the tray to stick onto the stilt (yes, I’m assuming the stilt body wasn’t the cause and will actually fire fine to cone 10 - I’ll amend this if it’s shown otherwise). As everything cools, they did so at different rates which meant they shrank at different rates as part of the cooling. The glaze tray is thinner, so it cooled faster and shrank faster. The tray applyed pressure on the stilt at the bottom as it shrank plus the weight of the bowl pushing down on the stilt in a different direction eventually all built up until the stilt snapped. Remind me - was the fuse on a single point? Along the full length of the stilt? Did the stilt look like it sank into the tray and become embedded? Or was it a clean even fuse?

Another much less likely one - the glaze tray clay body wasn’t rated for cone 6. I’d expect the whole tray to bloat and show other signs if this was the case though.

So a few questions I think we should look into if we really want to solve this:
Was the glaze tray a piece of bisque or had it previously been through a glaze fire before?
Do you know the clay body it was made from?
Was the glaze tray clean - what was the chance there was a coating of some type on it that caused the fuse to happen (previous drips, glaze dust from the space, something else, etc)?
Same for the stilt - was the bottom of the stilt clean?
Do you remember where in the kiln her piece was?
Can we run cone packs (a standard 5/6/7 setup) throughout the kiln during the next glaze firing to test for hotspots and see how the elements are doing?
Do you know the last time cone packs were run and what the results were?

Running witness cones on a regular basis is part of standard kiln maintenence. As many firings as ours is seeing I think having a defined kiln maintenence routine is important and keeps us ahead of a downed kiln.

But, back to stilts. The DMS owned ones, assuming they are standard ones and not specialty brands like Roselli or Bells, are meant to be used at bisque/low fire temperatures only. They can survive to 6. It’s the pins that are the main limiting factor on the firing range, not the clay base. The behavior you’re seeing from the DMS stilts is the exact behavior I’d expect to see from that style of stilt even if those stilts were brand new out of the packaging. The pins are not designed to support a heavy piece at cone 6. They will buckle (see my 1st post on why/how). There are ways to load balance a large piece with these and successfully fire them to cone 6, but it can be tricky and will probably still fail.

Firing the bisque rated pin stilts to cone 6 no matter how light the load, degrades the materials faster and significantly shortens the life expectancy of the stilt. You can do it and it’ll work, but there are several alternative methods besides using low fire pin stilts outside of their intended specifications. I haven’t looked through the DMS pin stilts. They may still be in good enough shape and usable for someone needing to stilt pieces for glazing at bisque temperature. For our glaze firings, we really need to look at getting a different method in place, whether it’s the all ceramic stilt, the specialty pin stilts, wadding or a combination so people have choices depending on their specific need and what they are trying to accomplish.

The spurs you mentioned, I don’t think they are worth testing because I don’t think we should be deliberately destroying them. Just stick to firing them up to cone 01 (2046) which is well higher than our bisque temperature. If used as designed, they aren’t supposed to wear out and can support any sized item. You just grab however many you need and lay out the support structure. Those are designed to minimize scarring. They are super handy to have. If no one is currently firing anything that requires their use that’s fine. Keep them available for when someone does need them and let everyone know we do have them available.

[dryad2b]

The stilt seemed to be completely stuck, although it had not sunk into the glaze tray. That tray had been used previously, although I don’t remember any glaze drips. Thus, it had already been fired to Cone 6. I tried to get the stilt loose from the glaze tray. It cracked in a couple of spots, and refused to come up.

The stilt had also stuck to the bowl. The pins under the bowl were completely mushed into the stilt body, and where the bowl touched the stilt, bits of it stuck to the bowl. They were pretty small bits.

According to the thermocouple temp, it only got to 2229, not the predicted 2232.