What are these "clinkers" that are left behind in the fireplace?

"Clinkers" is the word my dad came up with for them, because they make a glassy "clink" when you drop them. They aren't like regular embers, they're much denser, and they look kind of like coprolites made of particle board. Can't tell you what kind of wood makes them, because we use various types and they show up unpredictably.

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They ARE called "clinker" and they are the result of the fire being hot enough to fuse and melt the ash and non-combustibles in the fire. My day job is partly designing instruments to measure the properties of the liquid ash INSIDE the fire. Typical fusing points are around 1300°- 1400°C (2500 °F +), my instruments go up to 1750°C, 3200°F +.
CameronSS (author)  steveastrouk7 years ago
2500 °F? Most sources I find online are saying around 1200 °F-1500 °F...is it possible that it's just "sintering", instead of actually melting?
Are these wood derived or coal derived ? 
Break one open and see if the structure looks sintered. In the processes I am interested in duplicating, the liquid ash has to flow out of the hot coals, so we are usually well liquid.
CameronSS (author)  steveastrouk7 years ago
Wood. Like I said, it looks a lot like ashy particle board. They're relatively hard, but brittle.
You're right, they're not fully fused, but on their way. The stuff we make is glassy.
CameronSS (author)  steveastrouk7 years ago
Thanks!
Who knew we had an actual clinker expert on the boards? Cool.

Best Answer.
I just know how to melt it and measure it. The guys who understand what's going on leave me standing.

Steve
Goodhart7 years ago
RavingMad is pretty much on the money there.  As the fire gets hot enough, some of the wood and resin is burned with a lowered amount of O2 and as such, becomes very "similar" to what blacksmiths and forger's called COKE.
Coke was HARD and also hard to burn, but once it WAS burning, it gave you the hottest fire; so it was a desirable addition to the smithy's forge.

Your clinkers are more then likely  "pre-coke" though. 
Oh and I acknowledge Steveastrouk also with his answer.....
milkywaybar7 years ago
they are  a very valuble mineral called flausanite. they can be worth 100's of dollars for a 2 inch long piece.
They're just an incombustible byproduct composed of burnt resin, ash and dirt. I was told they were caused by reheating old ash, or too much dirt on the logs, or very very pitchy logs, or some combination of the above. Can't verify the accuracy of that part, though.
=SMART=7 years ago
Diamonds !!!!!!!!!!!!!!!!!!!