4 or 8 pound

JTknives

JTknives

Blade Heat Treating www.jarodtodd.com
Knifemaker / Craftsman / Service Provider
Joined
Jun 11, 2006
Messages
8,633
i have gotten mixed responces regarding the density of koawool. some say that the 4 pound is better because its lighter and will conduct less heat. and others say the oppeset meaning i would need twice as much 4lb as i do 8lb. where is the truth
 
because im finishing my forge and i found a full box of 4pound koawool for 60 bucks
 
Dan Zawacki said:
The folks I've been in contact with from suppliers always told me that the higher density insulates better. On the other hand, Thermal Ceramics datasheets doesn't differentiate the thermal conductivity by density. I would wager that the difference is likely miniscule.

http://www.thermalceramics.com/upload/pdf/514-205.pdf
Click to expand...

It does differentiate by density. There is not data for 4 or 6 pcf at 2000°F.
The difference in thermal conductivity ranges from 20-40% higher on 4 pcf vs 8 pcf, from 500 to 1800 °F.
Thanks for the link.
I had been wondering that myself, as I have a box of 4 pcf Kaowool...
waiting for me to quit procrastinating...:(
 
I used some of the 4lb stuff that I got a good deal on and had to reline my forge quite a bit. It just didn't hold up as well for me. Go with the 8lb from Darren Ellis. It's known good stuff and there's no surprises.

-d
 
The higher density wool is firmer, and thus holds up better. With rigidizer, 1/4" of satanite, and ITC-100, it will last a long time.
Stacy
 
After doing some looking around, it seems the thermal conductivity is related to the air space in between the fibers (as it is in home insulation), but is more related to reflectivity of heat.
The lowest conductivity is with the 8 pcf blankets. It goes up when you get get lighter or heavier than that density.
Here's a chart:
Thermal conductivity °C (W/mK) ,ASTM C201
Kaowool or Isowool
4 pcf 6 pcf 8 pcf 12 pcf

mean 260°C 0.08 0.07 0.06 -
mean 400°C 0.13 0.12 0.07 0.09
mean 538°C 0.19 0.15 0.12 -
mean 600 °C 0.21 0.17 0.12 0.14
mean 800°C 0.30 0.25 0.16 0.20
mean 816°C 0.33 0.25 0.21 -
mean 982°C 0.43 0.32 0.26 -
mean 1093° - - 0.30 -

Now to go smush my 4 pcf to a 8 pcf density... :D
 
Last edited:
At one point in time I tried to save a few bucks by buying a case of #6 kawool. What I really did was waste my money. The #6 (as has been mentioned) is not nearly as dense as #8, which means it will tear, shred, and burn out more than twice as fast as #8. Generally a case of #8 will last me about a year, whereas the case of #6 was gone in about 4 months.....for me it just isn't economical to use #6, so I can only surmise that #4 would be like using cotton candy. Like the old saying goes "pay me now, or pay me later."
 
personally, i think the soft insul brick work best, for durability, and there easy to fit, and heat up at about the same rate. only my opinion, i have no data to back this up, only what iv'e observed in my own shop.iv'e had both materials available to me for a couple years at no cost, and have chosen the insulbrick over the wool. i also make soup out of the 3000deg. cement and dip the insul brick before putting them in. i believe there pretty costly if you buy them from a supplier though.
 
I know this advice will probably be ignored, but I'll say it again anyway ;):D....

If you're using the forge a lot, ditch the wool and go with firebrick and/or castable refractory.

Ceramic fiber is not something you want to be around a lot. It goes in the air little by little as it degrades and you breathe it--especially when you rake it with your workpiece.:eek: It's on the CDC site as a 'probably human carcinogen.'



The heavier brick and cast refractory also serves as a 'thermal mass'. It stores up heat and can radiate it back for a long time. This is really convenient when doing more delicate pattern welding because it allows you to turn the gas/air way down and maintain welding heat in a much gentler atmosphere. (vs. blow torching your joints just to keep the heat up.)

That's my 2 cents. :)
 
JCaswell said:
I know this advice will probably be ignored, but I'll say it again anyway ;):D....

If you're using the forge a lot, ditch the wool and go with firebrick and/or castable refractory.

Ceramic fiber is not something you want to be around a lot. It goes in the air little by little as it degrades and you breathe it--especially when you rake it with your workpiece.:eek: It's on the CDC site as a 'probably human carcinogen.'
Click to expand...

That's why mine has 1/4" of Satanite all around and 3/8"-1/2" of castable at the bottom :) The only time I worry about fibers is during a reline and I wear my respirator then.

The heavier brick and cast refractory also serves as a 'thermal mass'. It stores up heat and can radiate it back for a long time. This is really convenient when doing more delicate pattern welding because it allows you to turn the gas/air way down and maintain welding heat in a much gentler atmosphere. (vs. blow torching your joints just to keep the heat up.)
Click to expand...

Just to play devil's advocate some more, the downside to that extra thermal mass is that it takes longer to get heated. If you're working a lot, for extended periods of time, castable is indeed great. If you're like me and get an hour here and an hour there, waiting 30 minutes to come up to heat sucks. I can be to welding heat in under 10 minutes with my "hybrid" forge. If you place the burner inlet right you're not torching your workpiece either.

Ok, enough devil's advocate for me...I gotta get to work. :)

-d
 
deker said:
... the downside to that extra thermal mass is that it takes longer to get heated. If you're working a lot, for extended periods of time, castable is indeed great. If you're like me and get an hour here and an hour there, waiting 30 minutes to come up to heat sucks. I can be to welding heat in under 10 minutes with my "hybrid" forge. If you place the burner inlet right you're not torching your workpiece either ...
Click to expand...

Well it's true it takes a good 20 min to get to heat and somewhat more to get good and soaked (I use natural gas), but the benefits of being able to, say, weld an edge around a viking sword with the forge turned almost off is pretty hard to beat.

For welding stacks in the press, or twisting/coining billets, it doesn't matter.

If you're doing long sections by hand that require multiple heats (on a piece that you're already into for 20 hrs or so)--I don't know about you, but I'll take every edge I can get. Having done it both ways, I'm here to say, this is an important consideration. :)
 
now say some one wanted to do a cast forge. of say 14 long by 8 accros with an 4" chamber. would the 2" wall thicknes be a problem and take a very long time to heat up. i was thinking forced air but i think i will go with 2 3/4 burners as i dont want to have to use power.
 
i think even if you used light weight castable, it would take forever, to heat up, and with the price of gas i see no benefit to not using forced air.

jt, if your interested, i got one that's 16'' long and 8'' round, with a hinged front door, and a 2'' k-wool liner, and a insul brick back door, if your interested , PM me and i'll give ya the details, it's never been fired.

i'll take $ or trade, you pay shipping., this is not an advertisement, just trying to help jt out.
 
It does differentiate by density. There is not data for 4 or 6 pcf at 2000°F.
The difference in thermal conductivity ranges from 20-40% higher on 4 pcf vs 8 pcf, from 500 to 1800 °F.
Click to expand...

Y'know, I looked at that chart for like a half hour like a monkey doing a math problem before I found what was right in front of my own face. :rolleyes:
Anyone wanna help me pull my foot outta my mouth?

Anyhow, JT - if the 2" is going to be just wool, no, the wall thickness shouldn't be a problem, but if the interior is a 2" thick wall of castable, then, yeah, it's going to take a long ass bloody time to heat up. The inside will be hot enough to forge in, but the castable won't be radiating its heat inward as nice and even yet until it gets a good glow on. You'll burn a lot of gas forging in something like that, but if you're welding a bunch of large billets and going to be at it for a long time, it might actually save you fuel, because once it gets up to temp, it'll want to stay that way a good long while too, and you can keep it good and hot at an idle for a good long time.

I had a forge with a 1" wall of castable, insulated by 2" of 8lb Kaowool around the outside of that that I had wanted to use for welding, but I gave up on the beast after a 45 minute burn at full bore didn't get the liner up to temp:eek:

I'm not yet welding in my current forge, but it's built for it, with a bubble alumina refractory lining the inside, surrounded by 2" of 8lb "superwool" (the less nasty to your health version of Kaowool), all lined with ITC100. I've slagged some metal in it, and coated a bar of steel with borax and melted the borax in it just to see how it would react, and it hasn't even noticed yet.
 
Well here's my forge for reference.
As you can see, it's a Franken forge. The burners are a modified Mankel manifold. The rest I designed for the kind of work I do and for easy maintenance. :thumbup:

As you can see there's lots of cast refractory (4-5 inches top and bottom) and the 'walls' are firebrick that can be arranged for the requirement of different welding situations (shapes, sizes, etc).

This is running on natural gas and is ready to weld from cold in about 20 minutes. It's fully soaked and can weld at idle (almost off) in about 45 minutes.

I don't think that's outa line. Plus, natural gas is much cheaper than propane (at least around here). I can use the forge heavily and it doesn't effect my gas bill (they bill in tiers and it never is enough to push me over to the next tier so it's effectively free.) :thumbup::thumbup::D
 

Attachments

  • forge.jpg
    forge.jpg
    37.7 KB · Views: 22
if i wanted to use natural gas would a gas line that feeds a range be enough for a forced air. i have an electric range but it has a natural gas hookup behind it.
 
JTknives said:
if i wanted to use natural gas would a gas line that feeds a range be enough for a forced air. i have an electric range but it has a natural gas hookup behind it.
Click to expand...

It might, but because natural gas comes in at such low pressure, you need to be able to make up for it in volume. Best is to run a separate line (1-inch pipe) right off your meter, directly to the forge (with shut-off valve, of course).

Depending on your burner arrangement, you want things to generally be bigger. If your manifold has jets, they should be drilled out to a larger size (30% or so would be a good place to start).
Most burners are just a pipe with a gas inlet. and are plenty big enough. I run three salt pots on NG with the simplest manifolds/burners ever. It's a 1 1/2 pipe with a 1/4 inch brass fitting with flexible line feeding right into it, a fan on one end and the burner on the other end is a step-down pipe fitting (1 1/2 to 1-inch) It's the salt-pot burner design on Don Fogg's site. These could reach welding temp on LP or NG without modification.


Using NG will save you a boatload of money and you won't lug bottles, freeze bottles or run out of gas at the worst-possible time.
It's not quite as hot as LP, so it takes a little longer to heat initially, but it's not that big of a deal if you're running a little blower.
 
Last edited:
You must log in or register to reply here.
Back
Top