DIY Kiln Build - Asking all the WRONG questions in all the RIGHT (Hopefully) places...

[Kirbcheck]

Sorry for the following ramblings that are about to interrupt your evening.

Greetings and Salutations from Omaha Nebraska. I'm a Newb (literally joined a few minutes prior to this post). A few relevant threads from this forum were returned on many a Google search I've made looking for clues to building my DIY Kiln. The community seems incredibly knowledgeable and helpful.  Unfortunately, much of what I'm trying to understand goes over my head, and I could use some guidance on where to start and what decisions are necessary for my specific build.

The project:

The construction of a kiln is actually a smaller portion of a much larger project. The true project at hand is to build a Dobsonian Telescope with a 20" ~f3.4 mirror. I intend to build this from scratch including the mirror. I have done research and feel fairly confident in my DIY abilities to take on this project. However, to make the mirror I will need to Melt glass into a custom mold and then slump the blank over a second mold to achieve the parabolic shape.  Hence my need for a kiln. I have been keeping my eyes out for used kilns in my area and they are either 1. Too small or 2. Too expensive. So now I'm wondering if it would make sense to add "Building a kiln" to the list. 

Keeping it cheap but effective:

I'm well aware that digging into hobbies is always a sure way to break the bank. I can understand that many will probably have an urge to (politely I'm sure) provide me with a reality check when it comes to the finances of this project. Please refrain from doing so. I understand that while I hope to keep this kiln to under $200 that it may not be reasonable. Please just humor me, while providing the best advise you can give on the subject of building a kiln. To keep cost down, I'm researching all steps in the process. Making the bricks from scratch, coiling the wire myself. Building a DIY Arduino Controller. 

My Needs:

To melt Glass into a cylinder 20" in diameter and 3 inches tall. I've attached a firing schedule I've found online from someone making mirrors in the same fashion I choose to. 

Available power: 240V - 50 Amp Breaker

Kiln size: 24" diameter, 12" tall

My Questions:

1. Is it reasonable to assume I can build a kiln of the size I need, that will maintain temps hot enough to melt the glass for my mirror with the attached firing schedules using my available power? (Don't consider cost).

2. Where do I start with the engineering of this project? 

3. What do I need to know??? 

4. I'm most confused with gauge of wire needed for size of kiln to attain my desired temps. Help!

Final thought:

I'm super defiant and urging me to do anything other than proceed full steam ahead is a sure way for me to continue down this path out of spite. 

 

[neilestrick]

Building your own electric kiln is not necessary. Plus you'd need somewhere to fire the bricks if you make your own. It's just not practical. If you just need a kiln to melt some glass, then find an old electric kiln, pull out the elements, and fire it with propane. Search 'raku kiln' on the Googler and you'll get an idea of what I'm talking about. 1750F is not hot by ceramic standards, so it won't be difficult to reach that temp. Alternatively, you can build a simple kiln with steel mesh and insulating fiber blanket. The difficult part will be holding temp, but it can be done manually with a pyrometer and some diligence on your part. Or buy an old junky electric kiln for $200. Since you don't need it long term it won't matter if it's not in good condition. You'll need to build your own controller for it, but that's simple compared to building the kiln.

[Bill Kielb]

On 1/6/2022 at 5:42 PM, Kirbcheck said:

Sorry for the following ramblings that are about to interrupt your evening.

Greetings and Salutations from Omaha Nebraska. I'm a Newb (literally joined a few minutes prior to this post).

Keeping it cheap but effective:

To keep cost down, I'm researching all steps in the process. Making the bricks from scratch, coiling the wire myself. Building a DIY Arduino Controller. 

My Questions:

1. Is it reasonable to assume I can build a kiln of the size I need, that will maintain temps hot enough to melt the glass for my mirror with the attached firing schedules using my available power? (Don't consider cost).

2. Where do I start with the engineering of this project? 

3. What do I need to know??? 

4. I'm most confused with gauge of wire needed for size of kiln to attain my desired temps. Help!

Final thought:

I'm super defiant and urging me to do anything other than proceed full steam ahead is a sure way for me to continue down this path out of spite. 

 

 

So now I'm wondering if it would make sense to add "Building a kiln" to the list.
Likely not as obtaining a used kiln or renting space in an existing kiln probably might be cheaper, definitely easier and a good chance of yielding reasonable results with less trials.

I understand that while I hope to keep this kiln to under $200 that it may not be reasonable.

I think that is accurate, even if asssuming your time or lost opportunity to do other things is of no value, the price is likely to grow considerably.

To keep cost down, I'm researching all steps in the process. Making the bricks from scratch, coiling the wire myself. Building a DIY Arduino Controller

Building your own controller likely cost $200.00 on its own and making bricks and winding your own elements seems a bit like buying a car without power steering but definitely doable.  The arduino allows any sort of program that you might need.  Bricks probably can be replaced with ceramic fiber for this kiln. Buying any wound element the approximate wattage is also likely cheaper and certainly easier than buying wire, making arbors and winding equipment and winding it.

Researching all steps is not the same as researching for constructability and economy so making these things may not pass your economy of construction research.

Available power: 240V - 50 Amp Breaker 1. Is it reasonable to assume I can build a kiln of the size I need, that will maintain temps hot enough to melt the glass for my mirror with the attached firing schedules using my available power? (Don't consider cost).

This is plenty of power - look and compare the wattage of similar kilns and you will find that this likely can be done with 2500 - 4000 watts, especially if you use fiber instead of brick. I think very reasonable to assume this can be done.

2. Where do I start with the engineering of this project? 
You have started, gather the facts as you know and understand them:  power, insulating material, shape of kiln, construction method, element mounting method, etc…. And list what you think is necessary for procurement then maybe design sketches of the kiln based on your assembly abilities, interior space needs, number of anticipated cycles to be used and refinement based on thinking through this process.

3. What do I need to know??? 
Wattage, size, materials, kiln shape and expected construction practices, programming, basic electrical safety, simple thermodynamics…… you seem to have a basic idea, certainly enough to allow you to research and learn as you put your plan into effect.

4. I'm most confused with gauge of wire needed for size of kiln to attain my desired temps. Help!

Seems like cart before the horse, pick a kiln, pick a required wattage, (look at already made kilns), longevity requirements seems to be - not much - so buying ready made elements and gauge of wire and element loading is likely not something you need to learn really. If you are compelled to learn, research radiant element design, nichrome, Kanthal wire etc….

Final thought:

I'm super defiant and urging me to do anything other than proceed full steam ahead is a sure way for me to continue down this path out of spite. 

Sounds like a good learning experience on lots of levels. If you have the drive, desire, money and time it all sounds doable fun and educational - even if it never gets built.

 

 

 

[Kirbcheck]

Thanks to both of you! I appreciate the feedback that you both have provided. And honestly, I've been keeping an eye out on the used kiln market in my area for awhile. If it's in the range of affordability, it's just too small. I'm considering other options, but for now I am planning ahead as if building is my only option.

I'd like to run my design past the hive mind.

Box 24"x24"x12" with heating element in the lid and body. Total surface area is 2304". With a Watt to Surface area ratio of 2.5, that means I'd need 5760 Watts for my kiln. I rounded up to 7000 Watts since I have 50 Amps to play with.

Reviewing commercial kilns and coils, it seems that 14 GA. Kanthal Wire is the standard for this project. When I do the math it seems the surface load is incredibly high. After pondering a bit, I noticed the information for the coils states they're to be wired in parallel to obtain the full Wattage. It got me thinking, does splitting the coils into two equal runs in parallel essentially half the the Surface Load of the elements?

Another question about Fire Bricks. As I previously mentioned, I've been researching making my own. The seem to mix perlite, sand , refractory cement and water. Does this not provide much insulation? Would it be effective if I used this method and wrapped it Kaowool?

What other considerations am I missing?

Thank you!

[Bill Kielb]

Box 24"x24"x12" with heating element in the lid and body. Total surface area is 2304". With a Watt to Surface area ratio of 2.5, that means I'd need 5760 Watts for my kiln. I rounded up to 7000 Watts since I have 50 Amps to play with.

Seems fine for your needs

Reviewing commercial kilns and coils, it seems that 14 GA. Kanthal Wire is the standard for this project. When I do the math it seems the surface load is incredibly high. After pondering a bit, I noticed the information for the coils states they're to be wired in parallel to obtain the full Wattage. It got me thinking, does splitting the coils into two equal runs in parallel essentially half the the Surface Load of the elements?

use ohms law, pick a wattage and develop with available elements on the market. If you use similar surface loading as a commercial kiln it will last hundreds and hundreds of firings or more at low glass temperatures anyway. I think you have read some surface loading considerations and are stuck on that for some reason. All good things to learn but in the context of what you are doing not primary concerns in any way if you are using ready made elements.  Pick a wattage, Wattage is the energy you will need to drive this to temperature. Achieve it as evenly as practical using elements already wound. Kilns with more than one element all end up in parallel to achieve a particular resistance and therefore wattage to meet their energy needs. Here is an ohms law wheel you might find handy https://diyaudioprojects.com/Technical/Ohms-Law/

Another question about Fire Bricks. As I previously mentioned, I've been researching making my own. The seem to mix perlite, sand , refractory cement and water. Does this not provide much insulation? Would it be effective if I used this method and wrapped it Kaowool?

Things are insulated because of trapped air, designing around ceramic fiber is pretty easy, I would use it exclusively. Making your own firebricks doesn’t really have value in my mind for this project, plus firing the bricks is likely a thing which means you need a kiln.. fiber = Better insulating value, less density, lower element loading, pretty easy in fabrication. Seems ideal actually.

What other considerations am I missing?

Just reading what you are focused on, more research on thermal energy, basic thermodynamics, energy transfer by conduction for a simple sense of the shell losses. A clearer understanding of basic electricity and wattage as a unit of energy 

Your goal is to fire a mirror in the easiest most economical way. My sense is building your own kiln, controller, programming, building a suitably precise hump mold, etc….. is going to be a huge ask. Enjoy the journey and learn while doing.

 

 

 

[Russ]

9 hours ago, Kirbcheck said:

Another question about Fire Bricks. As I previously mentioned, I've been researching making my own. The seem to mix perlite, sand , refractory cement and water. Does this not provide much insulation? Would it be effective if I used this method and wrapped it Kaowool?

About insulating value of kaowool.. years ago i had a discussion with the rep at American Insulation about what i could do to retain heat within my kiln that all i had to do was wrap the brick in kaowool and my problem would be solved. It turns out that kaowool is a terrible insulator at lower temperatures  (below 1200f). He recommended that I LAYER refractory insulation and use a lower temperature insulator like rockwool which would start its insulating properties and a much lower temp (600f).  So the sequence would be like this.. firebrick-kaowool-rockwool-fiberglass in that order . that way in the beginning of the firing the kaowool and rockwool would not be insulative but as the temp raises the insulative properties of the rockwool and then the kaowool would kick in. Each layer protects the previous layer as the temperature rises.  In the early days of firing I tried using just plain old fiberglass insulation against the firebrick but it would melt into puddles of glass.

  Speaking from experience I know this works. My kiln is quite large compared to most 80cubic ft stackable interior .  Before the insulation it took 12 hours of hard stoking to reach c10. AFTER insulating it now takes 7 hours on average ,the quickest being 5 hours.

[Bill Kielb]

The performance of materials can be looked up in a table and they vary over a temperature range. Firebrick for example: https://www.northrefractories.com/product-detail/refractory-insulating-fire-brick/  ceramic fiber blankets for example here: https://armilcfs.com/blog/durablanket-lt-and-ltz/

My feeling is layering is not necessary for what you are doing, the fiber out performs IFB from start to finish. Plenty of Raku kilns out there built in the cheapest of ways using ceramic fiber making it easily to his working temperatures. Follow the tables, don’t get hung up on making the best kiln ever using space shuttle tiles or aerogel unless that is your primary goal.

[neilestrick]

The tricky thing about using fiber with elements is trying to mount the elements to the walls. That's why I recommended firing with gas. If this is going to be a temporary/ low use kiln, then going to the trouble of engineering bricks, elements, and controls is a lot more time and expense than is necessary. You could use a basic raku kiln design with steel mesh and fiber and buy a few IFB for the base, and fire it with a  a venturi burner and a baso valve using a 100lb propane tank. With a pressure gauge, valve, and pyrometer you can manually set the rate of climb and hold temp. Low expense, low effort.

[Kirbcheck]

Quote

Posted 3 hours ago

The tricky thing about using fiber with elements is trying to mount the elements to the walls. That's why I recommended firing with gas. If this is going to be a temporary/ low use kiln, then going to the trouble of engineering bricks, elements, and controls is a lot more time and expense than is necessary. You could use a basic raku kiln design with steel mesh and fiber and buy a few IFB for the base, and fire it with a  a venturi burner and a baso valve using a 100lb propane tank. With a pressure gauge, valve, and pyrometer you can manually set the rate of climb and hold temp. Low expense, low effort.

I understand the reasoning for your recommendation on the Raku Kiln. I don't think this is the right option for MY application. Telescope mirrors are very precise objects. Air bubbles need to be almost none existent. The firing schedule I listed has been trial tested by an experienced mirror maker to reduce air bubbles and stress fractures due to thermal shock. Maybe I haven't done enough research on the subject, but I don't get the feeling that with a Raku kiln temperatures can be controlled as precisely as this application requires. 

[Mark C.]

I think buying the mirror you would be miles ahead on costs and heartaches-check out ebay for some of them-not to costly at all.

Now this teloscope uses many mirrors to look back into time

https://www.jwst.nasa.gov/content/webbLaunch/whereIsWebb.html

[Kirbcheck]

17 minutes ago, Mark C. said:

I think buying the mirror you would be miles ahead on costs and heartaches-check out ebay for some of them-not to costly at all.

Now this teloscope uses many mirrors to look back into time

https://www.jwst.nasa.gov/content/webbLaunch/whereIsWebb.html

 Mark, I can appreciate your thoughts on buying a mirror over making. There are two considerations. First of all, the cost of a 20" primary mirror is Thousands of dollars. Too expensive. Also, I have a DIY spirit that won't let me do it. I have numerous projects I have completed that required similar tasks. I've built a CNC from scratch, a bandsaw, Rehabbed a lathe... This is just how I am.  

[Bill Kielb]

1 hour ago, Kirbcheck said:

Quote

I understand the reasoning for your recommendation on the Raku Kiln. I don't think this is the right option for MY application. Telescope mirrors are very precise objects. Air bubbles need to be almost none existent

Neil is right from an ease of build standpoint easy to get lots of heat from propane. Raku style kiln was intended to imply simple use of fiber. Again your ability to solve mounting problems using ceramics, wire, etc… your temperatures are very low comparatively so retaining these items is fairly simple. If you use a Rigid fiber interior also allows setting the elements in mechanical grooves. Still an easier alternative than casting your own bricks. Again much of this goes to your comfort level and imagination in constructing.

I do have a question regarding the necessary precision of the mirror and the hump mold which will establish the initial shape and precision. Will this need final grinding, polishing etc…..?

[neilestrick]

@Kirbcheck Is there something special about the glass itself that requires you to make the glass, or can you buy a piece of glass that would work? If you can buy the glass, then I would look into renting kiln time for the slumping rather than building your own kiln.

[Kirbcheck]

3 hours ago, Bill Kielb said:

Neil is right from an ease of build standpoint easy to get lots of heat from propane. Raku style kiln was intended to imply simple use of fiber. Again your ability to solve mounting problems using ceramics, wire, etc… your temperatures are very low comparatively so retaining these items is fairly simple. If you use a Rigid fiber interior also allows setting the elements in mechanical grooves. Still an easier alternative than casting your own bricks. Again much of this goes to your comfort level and imagination in constructing.

I understand where you guys are coming from in terms of ease of build for a kiln that you assume will only be used once. However, mirror making is a bit of an experimental phase for me. I am assuming that it will take plenty of trial and error to where I'm happy. I see this kiln getting used many times throughout this process. Also, if my experiments in mirror making are a success, I foresee at least 3 more mirrors being made for siblings and parents. Also many people with large/fast mirror telescopes will build smaller mirror scopes for planets and everyday backyard viewing. 

As for making my own bricks, I feel confident I could make them provided I felt the composition held temps well enough to get my kiln to the necessary temp. That said I'm not sold on that idea so I'm looking into other options (Ceramic Fiber board, etc.) 
 

3 hours ago, Bill Kielb said:

I do have a question regarding the necessary precision of the mirror and the hump mold which will establish the initial shape and precision. Will this need final grinding, polishing etc…..?

Yes. Final Grinding and Polishing will be required. By slumping the blank to a given focal ratio, I can eliminate most of the heavy work though. I can also reduce the weight considerably with the proper blank mold.

2 hours ago, neilestrick said:

@Kirbcheck Is there something special about the glass itself that requires you to make the glass, or can you buy a piece of glass that would work? If you can buy the glass, then I would look into renting kiln time for the slumping rather than building your own kiln.

As mentioned above, there is plenty of this project that will be trial and error with the mirror blank making. The convenience of not having to pick things up and drop them off is part of it. I would assume that with a dozen trials will make it more economical to do myself. Finally, part of the fun for me is in doing everything myself from the ground up. Yes it causes headaches, but I find it an exciting challenge to make the tools I'm using to build the final project. Drives my wife crazy though.

[Mark C.]

3 hours ago, Kirbcheck said:

 Mark, I can appreciate your thoughts on buying a mirror over making. There are two considerations. First of all, the cost of a 20" primary mirror is Thousands of dollars. Too expensive. Also, I have a DIY spirit that won't let me do it. I have numerous projects I have completed that required similar tasks. I've built a CNC from scratch, a bandsaw, Rehabbed a lathe... This is just how I am.  

Ok I get it I have been into ceramics about 50 years now with  say 12-15 kilns built and have yet to master it (not enough time in ones lifetime to get there). I have not even started to master glass making other than high fire glass called glaze. I do know enough from my glass friends thats it also an art to itself and the coating of a mirror is also a specaility skill. I get that doing it yourself is what you want to do. How about working say with a glass person who has the tools to do what you need already-off him some CNC time in trade. I even know a glass blower in Kansas so they are around. Unless mastering kiln making as well as glass making is the aim and I get that but think decades for sure.

The tool rebuilds are all great and show that you are handy as heck and heck that discribes me as well.I'm just trying to shorten the process for you.Trading with a glass person seems a good compromise to me.

Keep us  up to date on this project as it out of the box for most of us.

 

[Bill Kielb]

2 hours ago, Kirbcheck said:

I see this kiln getting used many times throughout this process. Also, if my experiments in mirror making are a success, I foresee at least 3 more mirrors being made for siblings and parents. Also many people with large/fast mirror telescopes will build smaller mirror scopes for planets and everyday backyard viewing. 

I see a fiber build as thermodynamically superior for your use and done reasonably especially with rigid inside and out if necessary easily capable of 200 - 500 firings before an element change is needed. I don’t see a brick solution as being superior actually for your use, nor does it seem easier to create. This started out as potentially a kiln for $200.00 hoping to get a proof of concept mirror. Funny how things creep. It will be interesting to follow your progress, please post as you go, It may help others along the way.