Ceramics in a sauna?

[cdub]

I'm a product designer developing a line of water vessels to be used in traditional Finnish sauna. The intent is to come up with alternatives to the traditional wooden sauna bucket and ladle. I haven't done any pottery or ceramics  work myself, but it seems like a material worth investigating for this application. My main concerns are durability, and how hot the material would feel for a user in a 200 °F sauna.

My initial thoughts (all of which may be incorrect, so forgive me):

• A relatively lightweight, porous material like terracotta would feel cooler to the touch, depending also on the temperature at which it's fired.
• A porous container that slowly leaches water would be acceptable in this application, since the sauna is already a wet environment -- and that water might help keep the vessel remain cool to the touch.
• Glazes, being of a higher density and therefore more thermally conductive, would feel hotter. 

Does anyone have any information that might be helpful as I start this exploration? Specifically, I'm interested in the types of clays that would be appropriate here, durability relative to firing temperature, and the possibility of glazes or other surface treatments that would not leave the vessel too hot to handle at 200 °F.

Thank you for indulging someone who knows almost nothing and wants to know everything.

[oldlady]

since some of us have never used a sauna, it would probably help to explain just what you want and how it is actually used.   i know you pour water on hot stones to make steam.  that's all.

[Chilly]

If I understand correctly, you are asking:

Touch a piece of wood, then touch a piece of ceramics.  If both have lived in the same room for a period of time, one will feel colder than the other at normal "room temperature".

If when they get hotter, does that relationship of "feel"change?

Is it likely to change sufficient that a ceramic pot could cause injury or merely distress?

 

[Bill Kielb]

9 hours ago, cdub said:

My initial thoughts (all of which may be incorrect, so forgive me):

• A relatively lightweight, porous material like terracotta would feel cooler to the touch, depending also on the temperature at which it's fired.
• A porous container that slowly leaches water would be acceptable in this application, since the sauna is already a wet environment -- and that water might help keep the vessel remain cool to the touch.
• Glazes, being of a higher density and therefore more thermally conductive, would feel hotter. 

Here is my best take. Saunas can get up to around 200 degrees, not very survivable for humans actually, so how do they work? Humans can tolerate it because the relative humidity is very low which allows the body to perspire and the evaporation of same removes heat from your body enough so to keep your core temperature below 100 or around 100. If you stay in too long, bad things happen as you run out of cooling fluids.

So a persons skin temperature is likely around 100 or even below significantly. Regardless, it’s not 200 so picking up anything that has warmed to 200  results in heat transfer from the hot thing to your hand. The more heat retained by the object (Mass) and the speed it is transferred (again basically density) the hotter it will feel. Ceramics in this case does not appear to be superior to wood in any way in this instance. A wood vessel with a very light insulative  handle would be a best fit of desired characteristics. Again, not ceramics.

[neilestrick]

Aside from the heat issues, if the ceramic piece was to fall and break, you've got a dangerous situation for anyone in the sauna. Ceramic shards and bare feet don't mix. For me it falls into the same category as pools and hot tubs- no glass or ceramic beverage containers.

[cdub]

On 3/5/2021 at 11:08 AM, oldlady said:

it would probably help to explain just what you want and how it is actually used.

My apologies, I should have been more explicit. Water is used in different ways in a sauna, but it's most commonly poured on the hot stones to create steam. Traditionally, this is done with a coopered wooden bucket and carved ladle, but wooden buckets do not last long in the thermal and moisture cycling that occurs in saunas. I'm curious in exploring ceramics (presumably in combination with wood) as a more durable alternative, and one without the problems of offgassing that come with plastics, for example. 

[cdub]

On 3/5/2021 at 4:20 PM, Chilly said:

If when they get hotter, does that relationship of "feel"change?

Ceramics, being more conductive than wood, will feel hotter in a sauna. My intent is to combine wood and ceramics to best exploit the properties of both.

[cdub]

On 3/5/2021 at 6:13 PM, Bill Kielb said:

Ceramics in this case does not appear to be superior to wood in any way in this instance. A wood vessel with a very light insulative  handle would be a best fit of desired characteristics. Again, not ceramics.

Maybe I should have been explicit about my intent. I understand that any ceramic vessel in a sauna will feel hotter than a wood one, so I'm planning on combining ceramics (to hold water) and wood (to hold the thing that's holding the water). It is possible that a user could inadvertently touch the ceramic portion of the vessel, of course, which is why I'm trying to find out more about how ceramics would likely perform in this environment, in the hopes of mitigating the discomfort. We use and handle ceramic teapots, after all, and tea is commonly brewed at temperatures higher than those found in saunas.

I believe ceramics would, in fact, be superior to wood in that a ceramic vessel would be less susceptible to the thermal and moisture cycling that occurs in a sauna. I've made and used a number of traditional coopered wooden buckets in saunas over the years, and they invariably fall apart in the high heat/low humidity environment. Most sauna users ultimately revert to cheap plastic buckets (or galvanized buckets with wood-handled bails) and I think there may be better alternatives.

[cdub]

On 3/5/2021 at 7:48 PM, neilestrick said:

Ceramic shards and bare feet don't mix.

I think the concern about glass near pools and hot tubs is greatly exacerbated by the possibility of glass fragments entering the water; that possibility doesn't exist in a sauna. I don't think dropping and breaking a ceramic vessel in a sauna would be appreciably more dangerous than dropping and breaking a ceramic vessel in one's kitchen.

[Bill Kielb]

20 minutes ago, cdub said:

Most sauna users ultimately revert to cheap plastic buckets (or galvanized buckets with wood-handled bails) and I think there may be better alternatives.

Maybe, as a liner it likely is superior. Obviously it’s a little less thermally conductive than metal. Saunas are wet bulb (evaporative) environments, teapot handles cool because they are in cool air. Saunas, are very warm but low in humidity so humans can cool through evaporation. Put a teapot handle in 180 degree air, it cannot perspire so it will feel like it’s 180 degrees ........ very quickly. Your question of would ceramic  at 200 degrees feel too hot to handle, the answer is yes. Humans scald at 140-150f

[cdub]

4 minutes ago, Bill Kielb said:

Put a teapot handle in 180 degree air, it cannot perspire so it will feel like it’s 180 degrees ........ very quickly. Your question of would a ceramic  at 200 degrees feel too hot to handle, the answer is yes. Humans scald at 140-150f

I'm afraid it's not as simple as that: wood can't perspire, either, and yet I can comfortably hold a wooden ladle at 200°F. Our perception of heat and cold is a function of a material's conductivity and specific heat, and not merely its temperature.

My specific question, then, becomes how factors like the type of clay, the type of glaze, and the firing temperature might affect conductivity and specific heat.

[Bill Kielb]

1 minute ago, cdub said:

I'm afraid it's not as simple as that: wood can't perspire, either, and yet I can comfortably hold a wooden ladle at 200°F. Our perception of heat and cold is a function of a material's conductivity and specific heat, and not merely its temperature.

My opinion, this material is not ideal It will hold more heat than a low mass handle and yes the users hand will remove that  heat nicely cooling the handle. I believe it is not ideal, but you could heat up a teapot handle to 180 degrees and test out the perception thing.

[cdub]

4 minutes ago, Bill Kielb said:

My opinion, this material is not idea

Thanks. It is a challenging environment; I don't know if there is an "ideal" material. Some of the best-selling sauna buckets on the market currently are aluminum, which is arguably worse.

[Bill Kielb]

1 minute ago, cdub said:

Thanks. It is a challenging environment; I don't know if there is an "ideal" material.

Maybe lemons for lemon aid. Low mass, low thermal conductivity helps as a starting point on your search.

[cdub]

1 hour ago, Bill Kielb said:

Low mass, low thermal conductivity helps as a starting point on your search.

I'm curious if you'd offer the same advice to someone interested in specific clays and glazes for making teapots: by the very same criteria, ceramic isn't "ideal" for that application either, but it's still widely used. What's the distinction?

[Hulk]

It's all about the handle, yea?

[Bill Kielb]

2 hours ago, cdub said:

s still widely used. What's the distinction?

The handle on a teapot is cooled  by the ambient air (likely 70  - 100 degrees) your ambient is 180 - 200 degrees.  The  attachment  points on a teapot or mug are  relatively small  so cooling the handle with ambient air is a thing.  Also folks have found it generally acceptable and when not they grab with a dish towel so as not to burn their hand.. I guess I could measure handle temperature and quantify why it works but in constructing a teapot it’s very difficult to mix materials, but the fact is the thermal conductivity and retained heat has proven to be suitable with ambient air cooling in the case of teapots. . Some teapots have hinged handles partly because of design and maybe because they are cooler. I think if I were in your position I would look for materials more suitable, but only my opinion you do not have to agree.

This started out as your question about how would 200 degrees feel if it were ceramic. Respectfully, lots of  practical reasons were given why it may not be a good choice, including the possibility of breakage. They are just informed opinions Intended to be presented in a respectful factual way.  You might have a great idea, my opinion  however remains for the reasons already given.

[Callie Beller Diesel]

Given  the lack of any citeable research on this rather specific topic, the usual answer is to try it yourself and see.

 Make a few different versions and test them. My inclination, if you think porosity is going to be a virtue here, is to use either unglazed terra cotta, or fire one of those “cone 6-10” clay bodies only to cone 6. Either of those options should be durable enough to hold water, and will slowly weep. You might try a glazed one or two, just to see if there is any significant difference. I’d make the handles out of a different material, such as wood, rope or leather because a ceramic handle would likely get uncomfortably hot. Design them so the handles can be easily replaced if they rot like the wood does in that environment. 

I’m sorta curious about if it would indeed work, and if you do this, I’d love to hear back.

[PeterH]

35 minutes ago, Callie Beller Diesel said:

Given  the lack of any citeable research on this rather specific topic, the usual answer is to try it yourself and see.

 Make a few different versions and test them. My inclination, if you think porosity is going to be a virtue here, is to use either unglazed terra cotta, or fire one of those “cone 6-10” clay bodies only to cone 6. Either of those options should be durable enough to hold water, and will slowly weep. You might try a glazed one or two, just to see if there is any significant difference. I’d make the handles out of a different material, such as wood, rope or leather because a ceramic handle would likely get uncomfortably hot. Design them so the handles can be easily replaced if they rot like the wood does in that environment. 

I’m sorta curious about if it would indeed work, and if you do this, I’d love to hear back.

Loosely related paper on botijos - porous ceramics traditionally used for used for cooling drinking water in hot climates.
an ancient method for cooling water explained by mass and ...

 

PS Even containers for chilled snacks, drinks or towels
The physics of pot-in-pot coolers https://hal.archives-ouvertes.fr/hal-02408692/document

[cdub]

8 hours ago, Bill Kielb said:

I think if I were in your position I would look for materials more suitable, but only my opinion you do not have to agree.

Thanks for the feedback. I'm being a bit contrary, but your opinions are much appreciated. If I knew of a material that were more suitable, I would use it. In this environment, though, I'm not aware of any materials that work well across the board, so I think it's largely a question of determining which limitations are acceptable, and designing around those -- just as one does with a teapot.

[cdub]

10 hours ago, Hulk said:

It's all about the handle, yea?

It could be all about the handle, yes, but I think there are other ways of addressing the hot surface. One would be to select a clay that is low enough in density and conductivity that it doesn't get too hot, and another would be to form the piece with ridges, say, to reduce the contact area with the skin. I don't know if either of those ideas are feasible, so I'm looking for folks who might have more experience with this than I do.

[cdub]

1 hour ago, Callie Beller Diesel said:

Given  the lack of any citeable research on this rather specific topic, the usual answer is to try it yourself and see.

 Make a few different versions and test them. My inclination, if you think porosity is going to be a virtue here, is to use either unglazed terra cotta, or fire one of those “cone 6-10” clay bodies only to cone 6. Either of those options should be durable enough to hold water, and will slowly weep. You might try a glazed one or two, just to see if there is any significant difference. I’d make the handles out of a different material, such as wood, rope or leather because a ceramic handle would likely get uncomfortably hot. Design them so the handles can be easily replaced if they rot like the wood does in that environment. 

I’m sorta curious about if it would indeed work, and if you do this, I’d love to hear back.

Thank you! This is very helpful. I have done a little testing in my home oven. At 200°F, all the metals I've tried (aluminum, steel, brass, copper, and iron) are all very hot to the touch (not surprisingly). Thinner metals, like foil and beverage cans, can be handled, presumably because the thermal capacity is so low that only a small amount of heat is transferred before the object cools.

Glazed stoneware is cooler than metal, but still very hot. Unglazed stoneware can be handled, but is decidedly uncomfortable to the touch (I was able to hold it indefinitely, but only by passing it slowly from hand to hand). My next experiment will be to fill un unglazed terracotta pot with water and heat that; my hope is that the water will pull some heat out of the clay, and also cool it via evaporation, but we'll see.

The design I'm currently working on uses a wooden handle, so I'm less concerned about handling the ceramic directly than I am about discomfort or burns from the user incidentally touching the surface.  As I've mentioned on other comments, I see this piece as similar to a teapot: there are parts we understand we can touch, and parts we understand we shouldn't.

Thanks for the feedback, and the interest. I'll post more here as I learn.

[cdub]

1 hour ago, PeterH said:

Loosely related paper on botijos - porous ceramics traditionally used for used for cooling drinking water in hot climates.

I originally started working on this a few years back, and I think the botijo was what originally made me consider unglazed stoneware. Fascinating stuff, thanks for posting it.

[Hulk]

Different problem when the vessel is full of water vs empty; in the empty case, dampened material likely dries out fairly quickly in 200F dry heat, hence, cool-touch handle, imo, is got have. In the full case, how long until vessel and the contents reach ambient (200F)? At that point, methinks that handle is as important as for the empty and dry case, unless the evaporative cooling is so good the vessel just doesn't get that hot.

I also spent a few minutes perusing images of available containers marketed for spa use...

Vessels over a "certain size" - about a quart and a half for me, I have thumb damage - I want a handle.

[Bill Kielb]

1 hour ago, Hulk said:

200F)? At that point, methinks that handle is as important as for the empty and dry case, unless the evaporative cooling is so good the vessel just doesn't get that hot.

Making some form of evaporative cooler is a possibility and it might even work for a while, Evaporative coolers generally rely on air velocity and surface area and have been around forever in one form or another. Every cooling tower you see is an evaporative cooler which if operating well can depress the chilled water return to about 5-7 degrees over wet bulb temperature (70-75 degree return water) ,  even though it’s 100f @30% rh outside.

Having said that, there are many lightweight synthetic materials suitable and techniques to minimize mass, area, conductive contact area.

Making your own out of carbon fiber would actually be very easy as well and end up super light weight, super easy to overmold as a handle with minimal contact for connection points. You could even perforate it to make it lighter. Make a bucket rim guard the same way, overmold aluminum with silicon! Make it all aluminum, perforate as possible and overmold the contact points. A million possibilities when you try and design emphasizing on the things that solve or minimize  the thermodynamic problems at hand.

My suggestion: it’s usually not about what might be possible, it’s often more about the science and picking the properties of materials for your use. Understanding the requirements is most of the battle. From there designing  and trying things is the fun part.

I do this with young engineers daily, nowadays on energy use. Same principle, see the important facts clearly and then use your imagination to create something lasting and effective. Tonopah went bankrupt recently, just under a billion bucks I think, maybe more could be low by ten times actually.. It seemed possible but alas not really sustainable.