Cardboard is cheap/free, recyclable, strong, and if treated well, very durable. It can be fastened with glues or mechanical fasteners (drywall screws or rivets), and readily takes a few coats of polyurethane to create a harder, more durable finish. Most of these chairs were made from 100% waste cardboard. Cardboard dumpsters are plentiful on college campuses, especially behind cafeterias. Other good scavenging sites are big-box stores, strip malls, and appliance stores. You'll want to gather the flattest, biggest contiguous sheets you can find, mostly free of tears, water stains, and other weak spots. The other key is a box cutter and a lot of patience.
Step 1: Support Structure
This first chair is actually made from corrugated plastic campaign signs, held together with hot glue and epoxy. Campaign signs have great potential for Pop-Art designs. To make the support structure, I cut a regular pattern of slits in a series of sheets so they'd all notch together. Then, two top sheets acted as a floor plate does in a house, locking everything in place.
The second example is pure cardboard. Be sure to orient the corrugations so that they run vertically; you can see in the picture, the exposed edges (the bottom of the chair) are the ends of the corrugations that run vertically up to the seat. These tiny tubes or flutes are what actually conducts one's weight to the floor.