Homemade refractory as a barrier

[77katz77]

I currently have a Speedy Melt vertical propane forge I’ve been using for melting aluminum and am wanting to build a horizontal propane forge for blacksmithing. My plan is to use kaowool as an insulator. However, the steel bucket I have that I want to use as the outer “frame” is 13” ID I only want to use one 24” x 12” sheet of kaowool. For a 12” long forge I’ve determined that I need about a 7.5” diameter hole for the wool. Would I be able to use a homemade refractory (plaster of Paris, perlite, sodium silicate) as a way of filling space so the kaowool will fit nicely? I know homemade refractories certainly aren’t ideal but I’m thinking this might be ok because the kaowool is a main insulator.

[Buzzkill]

kaowool is not prohibitively expensive.  However, if you are set on only using one layer then you can use other things behind it to provide a better fit for your single layer.  It will be tough to keep something like perlite in place around the perimeter.  I've used it for the floor under fiber blanket before though.  Assuming the kawool does its job well you could probably get by with a layer of something like rock wool insulation behind the kaowool to take up the space. I believe that stuff is usually rated up to about 1200 degrees F, and you should have dropped the temperature enough with one inch of 8 lb. density kaowool so that it's not a problem.  I'm also assuming that you will be sealing the kaowool in place with an insulating castable refractory.

 

[BIGGUNDOCTOR]

Make a steel tube insert to reduce the inside diameter. The air gap will help keep the outer shell cool.

[Frosty]

Welcome aboard, glad to have you. If you'll put your general location in the header you  might be surprised how many members live within visiting distance.

No: POP, perlite, sodium silicate are NOT suitable for a forge liner. None of the ingredients will withstand the heat. However, Buzz seems to have read your post more carefully and I agree. You CAN use your perlite recipe as an insulating refractory liner between the Kaowool and forge shell. It's a serious PITA to install though. You could mow a couple lawns or rake leaves and earn plenty to buy enough 8lb. Kaowool to line your forge. 

You WILL want to add some fumed silica to your list so you can rigidize the ceramic blanket. Two good reasons for this: first is to help prevent small fibrous particles floating in your breathing air. Second is to make the blanket stiffer and a better foundation to support a hard refractory flame face.

Frosty The Lucky.

[Irondragon Forge ClayWorks]

If you haven't read this yet, I always recommend it to get the best out of the forum. It's full of tips like editing your profile to show location and how to keep off the moderators radar. READ THIS FIRST

If you have any HVAC shops near you, they have scraps of insulating wool that they just throw away. A dozen doughnuts and you will probably have more than you can use for free, just have to ask permission to raid the dumpster.

[77katz77]

So does regular fiberglass insulation work as well as/along with kaowool? 

I’m ordering what amounts to 12” 86” of kaowool (1” thick). This is enough for at least two layers of wool on the inside of my 13” bucket. I’m primarily going to be using the forge for knives and am curious if a forge with an ID of 9” , 12” length would be too big? Meaning that it would require a hotter flame/lose heat faster than a smaller forge. This is the reason I’m wanting to reduce the diameter in the first place.

[Buzzkill]

I wouldn't use fiberglass insulation behind only an inch of fiber blanket, but I don't know if it would be a problem or not.

It appears to me that you've determined the diameter of your forge chamber based mainly on the circumference of 24 inches of blanket.  You're also calculating based on the inside diameter of the cylinder that your blanket will make rather than the outside. To me that's not the right way to go about things, but if that's what you want to do then I suggest you go with a little smaller diameter.  The fiber blanket doesn't really stretch, so it's better to have to compress it a little at the ends to ensure a good fit at the seam where the 2 ends meet.

Obviously I don't know what your financial situation is, but a few linear feet of 24 inch wide 8 lb. density fiber blanket is a really good investment when it comes to gas forges.  Spending a few dollars more on the construction of a forge up front can save you a significant amount of money in fuel throughout the life of the forge.  If you are going to have kaowool shipped to you, typically it only costs a little bit more for shipping to get a fair amount more blanket delivered to your door. 

Some people who frequent this forum get all the fiber blanket insulation they need for free by making friends with HVAC folks who throw away remnants of rolls which happen to be large enough for projects like hobby forges.

You don't have to use the bucket you've chosen.  You can use some sheet metal or a smaller cylinder of some kind if you want to stick with those internal dimensions and then just use one layer of blanket.   Personally, I think you'd be better off designing the forge to fit your needs as well as you can determine those, and then obtain the materials to build that forge rather than let your preferred materials and quantities dictate the forge design.

 

[Irondragon Forge ClayWorks]

The insulating wool that HVAC shops use is like Kaowool although not always by that name. It's not fiberglass insulation used in insulating a house.

[77katz77]

1 hour ago, Buzzkill said:

It appears to me that you've determined the diameter of your forge chamber based mainly on the circumference of 24 inches of blanket.  You're also calculating based on the inside diameter of the cylinder that your blanket will make rather than the outside. To me that's not the right way to go about things, but if that's what you want to do then I suggest you go with a little smaller diameter.  The fiber blanket doesn't really stretch, so it's better to have to compress it a little at the ends to ensure a good fit at the seam where the 2 ends meet.

I have ordered what totals to be 12” x 86” of kaowool. The length of the forge is going to be 12” (I don’t think I’ll need it any longer). How would you suggest I wrap the wool? Disregarding what’s going to be on the outside of the wool. If I have a smaller ID for the forge I could get more layers but less volume , and vise versa.

Edited December 27, 2019 by 77katz77

[Irondragon Forge ClayWorks]

You might want to edit your post and trim that quote, as in this thread. The quote feature

[Buzzkill]

14 hours ago, 77katz77 said:

How would you suggest I wrap the wool?

This really depends a lot on what you plan to make in your forge and also how important fuel efficiency is to you.  If you plan to forge mostly small items and/or longer pieces that are fairly straight you can get by with a small short forge.   For larger items obviously you have to have openings that will allow those pieces to fit inside.  For long twists you'd need a forge that can uniformly heat up the length of the stock you want to twist.  If you can tell us more about what you'd like to do we can give more meaningful suggestions about the forge construction.

In general for a first forge I'd suggest trying to keep the volume of the forge chamber around 350 cubic inches or slightly less.  If constructed well at that volume then a single 3/4 inch naturally aspirated burner should be able to bring the forge to welding heat and the fuel consumption is reasonable.  When using hand hammers most of us can't work more than about 6 inches of material before the stock needs to go back into the forge for reheating, so a long forge tends to waste energy and can degrade the steel over time.  My preference is a shorter forge with larger openings.  Longer stock can be passed through the forge if generally straight, but you can't get oddly shaped pieces into the forge if any of the dimensions are bigger than your openings.

[77katz77]

Thanks everyone who commented. I have a MUCH better idea for how I'm going to build my forge and I've learned a lot.

[ThomasPowers]

Note that when asking questions you should deluge us with details---you know exactly what you mean by "I’m primarily going to be using the forge for knives " but we don't know if you will be doing production work with a half dozen blades in the forge at the same time 5 days a week; or making large bowies or even bush swords; or a couple of pocket knives a year.