I would not use any paper or cardboard that has a slick surface. Any other printed cardboard should be fine.

I know most magazines and some newspaper advertizing inserts are slick. I think a thin coating of clay is part of it. But is lets say a typical cereal box slick? Or a 6 pack beer tray? They look half slick to me. I tried an organic herbal tea box and the slicker label part of the cardboard seemed more resistant to decomposition. Even the white part of a square to go pizza box seemed to flake rather than decompose. Because we have massive recycling of everything under the sun in my town, I pull out only the luckiest bits of corrigated cardboard for my tiny herd. But I dream of the day they will be able to handle all the best bits of corrigated cardboard. It is ok with me if cereal boxes get recycled.

I recently harvested one of my bins. I did find pieces of cardboard that seem more or less intact. They had all been put through my paper shredder so they were maybe an inch long and 1/8" wide.

I have some old boxes of files that I plan to shred. A few months ago I shredded some old IRS records and put it into my compost pile. I suspect the inks my not be 'safe' by today;s standards but wanted to use them anyway. I have alot more (over 20 year old paper) that got stored so I may set up a test bin to see if the worms can handle those inks.

Like Equinox termed worms 'being like a canary in a mine' they can tell me if I have reason to be concerned using that paper.

Thanks all.

The slickness in much paper is perfectly harmless, and indeed beneficial, kaolin clay. But I do not know whether all of it is. My real concern is colored ink based on heavy metals.

I used to work in a print shop. When we did four-color work on slick paper, we often coated it with varnish. (Or was it lacquer? I can't remember.) In any case, I have found strips of un-decomposed cardboard in my finished vermicast, which I assume is from the few times I have put slick-surfaced cardboard in the bin.