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.
Step 2: Structure 2
The "neck" holding up the back was a little more complicated. To prevent it from tearing out or shearing back, it actually goes into the support structure all the way to the front edge of the seat. The seat's shape is derived from the tombstone-shaped pieces that were left over when I cut the arched ribs.
Step 3: Tube Structures
The first chair is made from tubes for the seat and backrest, and more tubes are buried in those side pieces to make the support structure. The corrugated cardboard is screwed into the tubes with drywall screws to make essentially a giant gusset that holds them all in place.
This second chair is made from shipping tubes. I wrote a whole instructable on this one, found here: FedEx Stool
Step 4: Hybrid Structures
For the cardboard cantilever chair, the six continuous pieces on the outsides were laser-cut from 1/4" chipboard. They sandwich a two-inch thick layer of corrugated cardboard. The inside pieces are not continuous; each consists of four striaght pieces interlocked and glued together. I have used many glues in these chairs, but plain old white glue or wood glue work the best, are the cheapest, and the easiest to clean up.
The corrugated cardboard supports the weight of the sitter, while the chipboard pieces act as gussets to prevent the back from tearing away as the sitter leans back. The cross-sections of the pieces also get thicker towards the center so as to put more support where there is more weight. Aforementioned printing tubes hold the pieces together without any glue, just friction. A few coats of polyurethane will extend the life span of the chair and give it a nice smooth finish.
Despite the fact that this chair is "paper", it achieves a version of the classic modern form, the cantilever. Cardboard is strong and versatile, as long as you work within the known confines of the material and find creative ways to deal with its limitations.
The lounger-type chair is a hybrid of masonite and cardboard, with the same organizing principles as above. Machine bolts both hold the layers together and penetrate the webbing to keep it from tearing out.
So, some ideas for all the cardboard out there: Look for boxes from certain stores and play with the graphics on the outside. Use tubes to make a conventional four-leged chair. Mix and match plywood, masonite, chipboard, tubes, and corrugated. Use corrugated plastic and rig a light inside so it glows.
Just watch out for papercuts and be careful with that boxcutter . . .