Making Glass and Ceramic Stirling engines

[Tom Booth]

Some interesting history.

Because of the ancient glacial clay deposits here, along the Mohawk and Hudson rivers in central upstate New York, settlers to this area sometimes took up brick making and there are several brick making factories down along the river.

The brick making trade required little capital. Basically a poor man's way of making a living from the dirt.

Where I am is higher ground far from the river but still within the zone of these glacial clay deposits.

Apparently glacial clay is the rarest clay in the world, characterized by it's super fine particle size.

I've had this clay/water trying to get it to settle in a bucket for I think about two weeks and it really just doesn't. The clay particle size is so fine that it just stays suspended.

There was a local brick manufacturing factory just a few miles from here down by the river.

[MikeB]

If the clay won't settle, I guess you'll just have to dry it like salt?

[Tom Booth]

If the clay won't settle, I guess you'll just have to dry it like salt?

The plaster of Paris is working very well.

I've already dried about seven batches on just this small piece of plaster: (back a page).

https://www.stirlingengineforum.com/vie ... 517#p20340

Another possibility, since the clay percentage is so High it could be dry processed right out of the ground. Just crush it to a powder and sift out any big rocks or gravel.

[Tom Booth]

Well, it had to happen sooner or later.

The plaster has soaked up so much water it has become heavy and waterlogged and, although the clay still appears to dry, it comes out gooey and sticky and tends to stay stuck to the plaster.

So, now I've paused making clay for a while as the plaster slab dries outside in the sun.

I did however get a pretty good yield of about 1 and 1/2 pounds of finished clay to work with. That should be more than enough for some experimenting and/or making a few things.

[Tom Booth]

I added a red dot to this map to show where the clay is coming from.

The green areas are clay deposits from a glacial lake that once covered much of the state:

Resize_20230908_134924_4944.jpg (145.73 KiB) Viewed 7842 times

The map is from this website:

https://www.nysm.nysed.gov/research-col ... -exposures

The area is a kind of high elevation flat plateau-like valley between the surrounding hills very remote, far from any towns or villages. Hard to imagine it was once the bottom of a glacial lake.

[Tom Booth]

I've nearly reduced the entire bucket of "slip" liquid clay to clay that can be modeled.

What I may do in the future though, is use plaster casting.

There are several Stirling engine types that could potentially lend themselves to being cast. Ones that are already designed to be cast in metal in particular.

Here is a pretty good video on the general ceramic plaster casting process:


https://youtu.be/8kVQFVgPbGc?si=UUHP7huCeK_OsCcF

It is not necessary to use 3D printing, but that could certainly be helpful

Here is another website with a lot of instructions and videos.


https://claybucket.com/3d-printed-and-slipcast-cup/

Of course, the methods demonstrated would have to be modified slightly for making engines or engine parts.

I did some plaster casting of ceramics back in shop class in high school, so I'm already somewhat familiar with the technique, though that was a long time ago.

What I need to do now is make some test pieces to see how much this type of clay shrinks.

[Tom Booth]

This type of Andy Ross style engine should be relatively easy to cast in ceramic using a two piece plaster mold.

Most of the main engine body parts that is anyway.

https://youtu.be/p6AMVxmI_8c?si=3pLT_27ulKCBgqDb


The choke or nozzle section of a thermoacoustic type engine, such as I recently turned out of wood should be easy enough to plaster cast in ceramic also.

I think there would actually be much less of, or no problem with overheating with ceramic vs. cast aluminium, on the theory that nearly all heat transfer that can lead to overheating is due to conduction rather than transfer by the working gas.

Less heat draining away to the highly conductive aluminum engine body should also result in higher power, if the same engine were cast out of ceramic rather than aluminum.

[Tom Booth]

I was trying to think of some method for making various precise engine parts from clay/ceramic.

After watching a number of videos on the various methods of working clay, a common method for smoothing pots and the like is to let the nearly finished work dry almost completely hard then use a knife blade or other sharp edges tool to scrape the surface down vary smooth, removing the rough, uneven outer layer.

This did not seem much different from turning wood or metal in a lathe.

I wondered if this was something that had been done by anyone before and I did find a few videos:


https://youtu.be/J3Opqb-sBro?si=bCgUygPCUDe8YMSu


https://youtu.be/n-7twF5_chU?si=faIpkqS0gCUShJDM

[VincentG]

That could come in handy for displacers. I've been able to mill concrete with relative success but never tried turning it.

[Tom Booth]

If anyone ventures into preparing their own clay, the general advice I've been seeing is to add 20% "grog", usually sand.

I had some "play sand" I had on hand so tried using that.

This turned out awful

My silky smooth clay became practically unworkable. Too many large pebbles in this "play sand"

If used, it is probably best to sift the sand first as it is far from uniform in size.

This is what I ended up with just sifting a few handfuls of "play sand" after basically ruining a small batch clay.

Resize_20230919_211534_4571.jpg (215.18 KiB) Viewed 7698 times

[Tom Booth]

I made a test cylinder of the local clay, without the addition of sand. It has been slowly drying and getting harder and apparently tougher.

I tried burnishing it, first with a stone, then a copper pipe, a piece of cast iron, the side of a nickel coin, just a fingernail, and here a brass nozzle from a propane torch.


https://youtu.be/C6A-CmjetgM?si=QHO1Nt-39K6SW1IO


I also tried a stainless steel spoon and the side of a plastic canister. I thought a fingernail worked well and did not discolor the clay. Possibly some animal horn would provide a nice polish, but just about anything smooth seems to work OK. The plastic, though, seemed to have a tendency to scuff up the surface more than polishing it at times.

The various metals seemed to impart some of their metallic luster.

Anyway, the clay has become surprisingly hard, especially for still being just unfired "greenware" which is often extremely fragile. This stuff is already nearly as hard as stone, but at the same time, with a kind of silky smooth quality, between hard plastic and the kind of hard rubber used for engine mounts.

My intention is to turn it on the lathe and then fire it in the kiln, but I'm tempted to try using it unfired, perhaps used as a piston, burnished with graphite.

I've never burnished clay before. An ancient technique for making clay cooking pots and such water proof. It works much better than I could have imagined, on this local clay at least, It feels almost like metal after burnishing, and in the unfired state at least, I can run it under water with no effect, unless I really rub the water into the clay hard.

[Tom Booth]

Tried turning the clay cylinder down on the metal lathe today.

Not making anything specific at this point, just wanted uniform sizes to test the percentage of shrinkage when fired.

Resize_20231011_201348_8966.jpg (122.83 KiB) Viewed 7614 times

The greenware turned very easily and came out OK considering I was pretty aggressive in the amount of material removed in a pass.


https://youtu.be/dJyD8hbSLb8?si=fjDHgA-JTgLdQOHJ

[Tom Booth]

Into the microwave kiln:


https://youtu.be/jcYoYVcvzc0?si=ZIsTVVkLOAxkLrLQ


Still don't know how it turned out.

Just waiting for the kiln to cool down, but as far as I could see in the white hot glare inside the kiln, I think it's still in one piece.

[Tom Booth]

The results:

https://youtu.be/_VxwuRN4lUE?si=Ty2EARqh9m2L9NZV


The clay fires surprisingly well in the microwave kiln!

I suspected this was a brick clay, since brick making in the area was a trade at one time but the change in color from grayish tan to a bright salmon was still surprising.

The clay held its shape very well and shrinkage was minimal measuring just 1.6% at the thicker end and about 3% at the smaller end or about 2.34% average which is actually better (less shrinkage) than most commercially available clay.

[Tom Booth]

A few interesting highlights from an article about ceramic pistons and ceramic use in engines generally:

...Ceramic materials have low thermal conductivity, and are often used to make combustion chamber parts such as pistons, cylinder liners and cylinder head bottom plates, and to insulate the combustion chamber. In ceramic non-cooling engines, even the independent cooling system of the engine is cancelled to prevent the loss of heat energy in the cylinder.

...Reduced weight of moving parts can bring benefits such as friction reduction, energy saving, faster response and reduced vibration....

...The hardness of high-temperature structural ceramics is much higher than that of metals, and the wear resistance at high temperatures is better than that of metals, ...

...Ceramics have a variety of excellent characteristics. When used in automobiles, they can effectively reduce the weight of the vehicle, increase the thermal efficiency of the engine, reduce fuel consumption, reduce exhaust pollution, increase the life of wearing parts, and improve the intelligent functions of the car.

http://www.nbceramics.com/amp/What-is-a ... 97992.html

What I find most interesting is that it is apparently recognized that eliminating the cooling system and retaining heat in the power cylinder improves engine thermal efficiency.

Generally speaking, the cooling system in heat engines is to protect the engines parts from heat damage not to improve engine efficiency or performance.