A guess could be that the original owner of the house just threw the coal ash from the furnace out into the yard. 

Many of the older (say pre 1940 or so) still have coal chutes on the basement walls for getting coal into the basement easily, where the coal-fired furnace or boiler was located. 

The 12" of overlying clay could have been fill brought in at a later date.

Back when coal was commonly used to heat homes, typically people produced more coal waste than garbage. I got to see a cool historical text recently showing the amount of garbage hauled from homes and the amount of coal ash hauled away. Coal won.

The 1918 Fire reached the east: The original Northland Country Club  clubhouse was lost in the fire.

Not that it means your charcoal layer has anything to do with that.

I'd be happy to come over and take a look when things thaw.

The fact that this is only 12" deep and in the city suggests to me this is not a natural charcoal deposit (more likely something someone buried.)

However, there are so few radiocarbon dates on the ancient glacial lake clays and shorelines  around here, I'll give any prospect a look.

I can be reached via email. I'm in the Natural Sciences Department at UW-Superior.

-Andy Breckenridge (geologist)

I've got science for any occasion
Postulating theorems formulating equations
Cheech wizard in a snow blizzard
Eating chicken gizzards with a girl named Lizzy
Dropping science like when Galileo dropped his orange

I can post if it comes to anything - but it's probably fill as some suggested. 

If it's not fill, there should be a nice soil profile. I'd expect a darker horizon close to the surface, and the clay should have a blocky texture.

.........

No (natural) sediment is acruing in Lakeside, but soil has formed on the 10500+ yr old lake clay. Dead plants are added to the top, and water and oxygen leaches down and interacts with the sediment. 

If this is dumped coal - someone covered the charcoal - which makes sense.

Tony D is likely correct if the house was along east burn area.  Are you in the lower Lakeside neighborhood?  Further, having assessed historic burn areas associated with old farm sites up closer to UMD from an environmental due diligence perspective, we've encountered many sites with elevated heavy metals and other not-so-good contaminants that should not be exposed to the kids.  Did you find anything suggestive of burning? (i.e. cans, bottles, rusty bolts, etc.?).  Have also worked on sites where foundry sands and ash were imported that were orders of magnitude more nasty.  Don't get freaked out, but don't be too casual either.

No, its mostly clean cut charcoal like ash.  But the d-bag who lived here before buried some nasty shit near the garage, which was quite an eye opener when I dredged it.  Andy, Ill get ahold of you in the spring, much appreciated.  I shant underestimate folk's interest in science from now on!

I live on 47th and Cooke and ran into the same thing in my backyard when digging piers for my sauna. A lot of coal and coal ash as well as lots of small bottles. I assumed that I was digging in the old backyard dump. House was built in 1891.