Biochar out of cardboard/ paper?

I'm wondering about making biochar out of cardboard and paper for use in a garden. I've heard some people say that there would be noxious substances in the ash. However, I'd only use paper and cardboard that I was comfortable composting anyway; and the stuff would be there one way or another! In fact, except for heavy metals, a lot of stuff might have burned off. I'd be using uncolored cardboard mostly. I'm sure the burn would go rather fast. Any other problems you can think of?

I've chucked paper and cardboard into biochar burns.  Sometimes you can still distinguish it after the burn, because that char ends up in thin flakes. There is really nothing bad that I can think of that would be left in that kind of char.  Even if you used glossy colored paper, which has a high clay content.  In fact, a little clay in the biochar might be an added benefit if you were adding it to a very sandy soil.  

Edited to add:  What makes good biochar burn starter material are cabbage boxes.  The paraffin on that cardboard board makes it light up like fatwood.  So the next time you are scrounging supermarket throwaways, cut those into pieces and use them.