Suitable rocks/stones for solid fuel forge

[J. Hamilton]

I have a bunch of these walkway/building stones I wanted to use for a small coke/coal forge but have heard using concrete could explode? 
here is a picture example of what I have.

if this isn’t good to use what type of stone or rock is ok to use? Other then fire bricks.

thank you for the feedback 

[ThomasPowers]

Why stone, why not  dirt/clay?  Did the 700 hits you get when you do a search with your browser  on: JABOD site:iforgeiron.com    not give you enough information?

[Frosty]

That is NOT stone, they're concrete pavers and  a B A D choice to make a forge from. Maybe if you had a reasonably thick layer of fire brick or packed clay between the fire and the pavers it'd be okay. 

As a rule of thumb. NEVER put concrete in or next too a fire. I don't care how many times you've seen a BBQ made from cinder block a BBQ barely counts as a fire and it's still a short lived use.

Frosty The Lucky.

[TWISTEDWILLOW]

5 hours ago, J. Hamilton said:

heard using concrete could explode?

Yes concrete will send pieces of hot shrapnel flying when I gets hot the moisture expands an can’t escape fast enough, that’s why even cinder block building collapse and crumble during a fire,

but but but!!!

Rocks aren’t much safer when it comes to hot chips of shrapnel flying off, 

I’ve seen solid stone split, crack and explode in a fire an shoot hot pieces of sharp jagged pieces flying, 

I agree with Thomas an Jerry! 

stick with steel, cast iron or clay firepots, the rest of the forge can be built out of whatever materials you want

but anything in the direct vicinity of the firepot needs dirt or clay lined, unless the whole forge is steel then you don’t really have much to worry about,

[Jobtiel1]

I started forging with an arrangement of hard fire bricks. certainly possible, but a JABOD would be much much better than that. I think you will getter a way better forge that way.

~Jobtiel

[Charles R. Stevens]

Realistically the only stone that withstands forge tempi tires is soap stone. I melt hard fire brick and red brick easily, and quarts will melt as well, so sand stone and granite will melt. Limestone truly sucks in the forge. 
 

you can buy cast-able refractory and it works great for making hearth stones. as dose steel. 
 

If you do some repercussions you will see that English smiths have used side blast forges that are boxes of fly ash and clinker with an inch or two of coal on top to store the days fuel (dost work with charcoal) and the green coal works to bank the working coke. 
 

but hey, get your welding jacket and safety shield out and try the cement pavers if you don’t believe the other guys. 

[Frosty]

Wear: safety glasses, cotton or wool clothes, pants that cover your LEATHER boots and a turtleneck to keep HOT spalls of concrete from going down your neck.

And Please PLEASE do NOT let children watch! Injuring a child is . . .. . 

Frosty The Lucky.

[TWISTEDWILLOW]

Yeah when I was a kid I learned the hard way not to throw chert an flint rock into a big brush fire! They Al an an will explode,

just lining rocks around a campfire isn’t dangerous because the temp never gets that hot that fast so the stones have time to release the moisture,

lots of old an new forges are masonry, made from brick or stone but you notice the firepot itself is either steel, cast iron or as mentioned it’s lined with something to keep the heat away from the stones or bricks,

[George N. M.]

To add to what the others have said the old geologist will add his 2 cents:  Most sedimentary and much igneous rock is unsuitable for a forge because of the porosity which may contain water which will equal steam explosions which equals hot fragments flying at high speed.  About the only suitable natural rocks are fresh (unweathered) high grade metamorphic rocks such as the soapstone Charles mentions.  They have been subjected to high temperature and pressure already in their geologic history and have nearly no porosity for water to seep in.   That said, outcrops of this type of stone are often weathered and have developed cracks and fissures that can contain the dreaded water.

You can also get cracking and fractures from differential heating.  One side of a rock or stone may get hot in a forge or campfire and expand a bit causing it to break from the cooler side.  This sudden fracture can be pretty spectacular sometimes.  Forsest fires can fracture boulders 10 or more feet across.  This was the way of much mining prior to explosives, get the rock hot then suddenly quench it to break it up.

So, generally speaking stone and rock, with certain exceptions, are not the best choices for a forge.

BTW, the melting point of quartz (SiO2) is about 3,000 degrees F.

"Byt hammer and hand all arts do stand."

[Frosty]

Granite has a reputation for exfoliating when heated. It exfoliates due to freeze thaw as well, it is I believe granite's main form of major erosion and why formations like half dome are big beautiful granite domes. There was a major exfoliation in Yosemite a few years ago killed a few people IIRC. 

Frosty The Lucky.

[Stash]

Frosty- the exfoliating granite  was used to my advantage  for many stone masonry projects. Example - capping steps or a stone wall with granite or bluestone flagging often times had fresh saw cuts exposed to view. Wearing goggles and mask, gloves and welding jacket, I would spritz the fresh cut with water and apply heat from a propane rosebud. Little flakes would pop off and give a nice pebbly texture. I had that done on an 11' long slab cut granite diving board  3' wide, it came out beautiful, as well as providing a no slip surface for wet feet. It was actually more of a jumping slab, cantilevered over the pool edge. In the stone world, they call the process 'thermalling'.

Steve

[George N. M.]

What many folk do not realize is that granite has porosity, water can penetrate little spaces and cracks between the individual crystals which make up the granite.  But it does not have permeability which means the pores and cracks are poorly interconnected so that it cannot pass water through the rock like you can with some sandstones.

This is what was happening with Stash's "thermalling," little steam explosions flaking off individual crystals from the surface.

The Fort Morgan, CO cemetery used very hard well water for irrrigation for years and it had the effect of damaging granite headstones by seeping inbetween the crystals and when the irrigation water dried it left behind Calcium Carbonate (CaCO3) crystals in the cracks.  As the csrbonate crystals grew they caused the granite to spall off damaging the headstones, to the point of obliterating some of the inscriptions.  Besides this the well water played havoc with domestic plumbing systems and appliances and was fairly nasty to the taste to boot.  About 25 years ago the city bought water rights and built a pipeline to bring snow melt and rain water water from the mountains to replace the well water.  A great improvement.

"By hammer and hand all arts do stand."

[Frosty]

A great deal of the work on Mt. Rushmore was done by thermal spalling. 

Frosty The Lucky.

[Lee188]

From what I have found all the old stone and brick forges where jbod forges. Colonial Williamsburg uses brick but it's just a box,I believe they are filled with ash. If you're plan was to use the blocks to make a jbod then you can use just about whatever you want as the fire is far enough away from the block. If you are wanting to build something small or have the fire right on the block listen to the other guys, no sense hurting yourself or others with spalls blowing off or doing all that work and have it not look how you plan answer a couple months.