Not much ash after burning?

[TresK3]

I've been burning a bit of wood for about 7 years now (about 1-2 cords a year) to supplement my heating. This year I noticed that after a burn there was some very hard material left in the firebox, under a bit of ash. The pieces are up to 6 inches square and flat; reminds me a little of slate tile. I can break this apart, but not easily. If I leave it in the fire box, it seems to burn, and I don't seem to be collecting as much ash as in previous winters.

What is this stuff, and why am I just now noticing it? My mix of wood hasn't changed much.

 

[begreen]

Ash accumulation depends on the wood species. The hard material is most likely just a clinker. Pretty common with high silica wood.
https://www.hearth.com/talk/threads/biggest-clinker.79475/

 

[Ram 1500 with an axe...]

When I burn red oak only, I get nothing but ash and nothing to help with relighting.....

 

[BrotherBart]

nothing to help with relighting.....

You sleep way too late.

 

[TresK3]

Thanks all. Definitely clinkers. I looked at the thread begreen listed, and some of the links listed from there; the photos were just like what I get. Never had them before, but I'm getting really good burning this winter, so that might be different. Having less ash makes sense too, if I'm getting more complete combustion. They seem to reburn - at least, if I leave them in the fireplace, they disappear and come back.