What is the difference between k26 and k23?

[Awrksmokey]

What is the difference between K23 and K26 firebrick? Is either better for gas forges? I'm curious, because I've heard both being used for kilns. (I think this is the right place to post this in, I'm sorry if I overlooked another section where this would fit better)

[Latticino]

The density, insulation value and maximum temperature rating are different between the two.  You can get details from any refractory brick manufacturer's website.  In addition, from my experience, the K23 brick is more fragile.  Personally I'm not a huge fan of brick forges.  For a gas fired forge you need a flame impingement face that can withstand up to 3,000 deg. F, IMHO, and neither of these will do it.  They also don't have as good insulation value per unit thickness as high temperature insulating blanket.  If I had to use brick for a forge I'd probably go with an inner liner of K26 (2"), then an outer layer of K23 (2"), with a split hard firebrick floor (1", preferably High alumina).  Or just a 4" thickness of K26, though the insulation value wouldn't be quite as good.  If the former design, be sure to stagger the seams.

[timgunn1962]

For any given manufacturer's products, the 2300 degF grade tends to be the better insulator, is lighter (less dense), and cheaper. For kilns, it is usually the better choice because the maximum kiln temperature tends to be less than "about" 2300 degF (less dense= lower thermal mass= faster heating. Better insulation= lower steady-state heat loss). 

For forges, 2600 degF-rated IFBs tend to be better and 2800 or even 3000 degF-rated IFBs "may" be better still, because the temperatures are higher. Even if the main forge temperature is below the rated temperature of the IFB, most burners tend to have parts of the flame that are hotter than others. 

IFBs tend to be a poor, or at least sub-optimal, choice for most forges likely to be built by anyone asking the question. IFBs are not usually good at tolerating steep temperature gradients and these are an inevitable consequence of the (very) rapid heat-up and (relatively) rapid cool-down most of us want from our forges. They tend to crack under these conditions due to differential thermal expansion and it does not take many cycles before they have cracked enough to crumble to something resembling coarse gravel.

Industrial forges constructed from IFB are usually used in industrial production where the forge heats slowly, stays hot for a very long time, then cools slowly, preventing the steep temperature gradients.

Despite their limitations,  IFB forges are very good for some things. I have built IFB gas forges in the morning, assembling them dry into an angle-iron frame, and been using them by lunchtime. I could not realistically do this with a ceramic fibre blanket forge unless I did not seal the fibres in. I'm in the UK and our climate means that drying times are tediously long, but I don't even think a decent refractory coating could be properly applied, set/dried and fired in a day by someone working in a desert. 

One thing that I have found is that the expensive, branded IFBs tend to be tightly controlled to optimize certain characteristics. Using cheap import IFBs, which are denser, poorer insulators and are marked as 2300 degF grade, I have had no particular problem running to much higher temperatures than their rating would suggest is wise. I would not use them for a kiln, but they do pretty well for a forge

The photos show the temperature measured in the forge (1545 degC is 2813 degF) and the effect this had on an off-cut of a KM23 IFB I had used to restrict the forge opening. The JM23s are the Italian-made equivalent of the US-made K23 IFBs and are from the same manufacturer. The JM23 melted into an impressively small puddle, given the volume of IFB lost. The rest of the forge was made from the cheapest, nastiest 23-grade IFBs I could get and survived pretty well (no melting at all). The cracking is visible in the photo. . 

 

[Frosty]

I think that about covers it. K26 are rated for 2,600 f. max working temps. There is a safety factor in the rating and it will withstand the rapid thermal cycling home or hobby forges undergo. However as Latticino and Guns point out it's not a good choice for a flame face. Propane flame is very chemically active and can hit temps of 3,200 f.. Even good kiln washes won't stop the shielded material behind it from being subjected to temperatures greater than they are rated for including the safety factor. 

A 1/2" layer of an insulating castable refractory like Kastolite 30 and a final kiln wash can make a survivable environment for IFBs adequately rated. Currently our club is experimenting with 6 brick K26 single burner forges with 1/2" Kastolite armor and Matrikote kiln wash. It hasn't been in use in our club long enough to have an empirical opinion, I'll post.

Used as insulating backers is the norm for ceramics, glass blowers and industrial furnaces. The K26 "ceramic tiles" are a relatively new comer to the market and may be a good alternative it's just a little early to know for sure. Ceramic blanket with a castable refractory inner liner and a high alumina kiln wash is a known effective forge liner system. It's a little more of a hassle but it we KNOW it works well.

Frosty The Lucky.

[Awrksmokey]

Thank you guys for the in-depth responses! I didn't realize kilns got up to a less high heat than gas forges. Well today I learned!