[Rob S.]

I have not seen it yet but a customer had us fabricate a ring made of 3 cm black galaxy.  The ring is 12 inches wide and surrounds a gas and wood fire pit and the customer says it has cracked.  My question is why, is it because of a specific mineral in the stone?  My guess is that soapstone is a better choice for this application but not sure if the customer will go for it.  I am told that the stone is about 3-4 inches from the gas heat source portion of the fire pit. Any help appreciated.

[Jeff Handley]

Rob,

This is from the Design Manual. There is currently no standard for pit style facings.
2.3 Backup Materials/Temperature.
Cracking of fireplace facings is primarily due
to expansion and contraction of backup
materials. Ensure that stone surface
temperatures do not exceed ability to safely
and comfortably touch the stone facing when a
fire is burning in the fireplace. Setting space
between stone and backup should be at least
¾”.

So, it could be the expansion of the material under the ring, or it could be thermal shock that caused it to crack, or a combination of both.

Soapstone may be your best option.

[Tom M]

Jeff,
Why doers Bluestone seem to work so well when other hard stones don’t?

[Jeff Handley]

Hi Tom,
Most of this is from some study I did on the ingredients of soapstone, the sciency info is from USGS.

Stones that perform better under high heat are dense & relatively homogeneous.
Soapstones and some bluestones may also contain Chrysotile. This is the mineral element that is better known commercially as the main ingredient in asbestos. Like Silica, precautions must be taken during the cutting process. Finished products are no danger to consumers as the mineral must be in a friable state (you need to break it apart and make it airborne for it to be a problem.)

As Rob described the problem, my first thought was the inside of the ring cracked away from the cooler outer edge of the stone. Granite can take a lot of heat, but it can’t take high heat in one area, because the cool areas are contracted and the hot area will expand.

[David Gerard]

Is the fire pit outside? If so could it absorbed water at some time and popped when the heat got intense? Maybe the substrate/foundation has flaws too. I know certain types of river rock will pop when used on high heat hearths and fire places…im told its from the water in the stone. Just my 2 cents.

[Tom M]

Thanks for the education, Jeff. I have to say I did not think that expansion/contraction was much of an issue with natural stone.

If more monolithic stones are better for this, would that include absolute black? That seems like there is only one mineral type in it and it is well compressed.

[Jeff Handley]

Tom-The Absolute black can perform as a fire ring, but not in a single piece configuration.

Rob- If you cut the Ab Black ring into segments, you will help alleviate the thermal gradient issues. You will definitely want to make a cut where it naturally cracked. That is where the intense heat expanded the stone unevenly.

[Jeff Handley]

Here is some technical information on the Thermal Properties of Stone.

http://www.alberenesoapstone.com/wp-content/uploads/alberene-soapstone-thermal-properties.pdf

Courtesy of our friends at Alberene Soapstone.

[Rob S.]

Thanks for all the input everyone, sorry I did not respond sooner, busy week.  The pit is outside and sits on a ring of concrete wall pavers built up about 4 layers high.  The customer is going to let us cut it into 4 sections.  Any suggestions on what to seam it with, I was thinking a black silicone to allow for flex, but will it withstand heat?

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