Kraft boxes and kraft board

Kraft paper is paper made by the kraft pulping process, in which wood chips are cooked in a chemical solution that dissolves the lignin binding the fibers and leaves the cellulose fibers long and intact. Kraft is the German word for strength, and the process is named for the result: the fibers come out longer and less damaged than other pulping routes produce, so the paper is stronger. It comes out brown because lignin residue and the cooking chemicals stain it, and bleaching it white is a further stage that weakens the fiber again. Kraft board is the same pulp formed thick enough to hold a crease and stand a panel up, which is what a folding carton is made from.

Natural kraft is unbleached through the whole caliper, so both faces are brown, the fiber is visible, and every printed color sits over brown. White kraft carries a bleached top ply over the same unbleached body, so the print face is white and the reverse stays brown. The practical difference is what the artwork can do: natural kraft warms every color and loses pale tints into the board, while white kraft prints close to proof. Natural is the stronger of the two at the same caliper, because the bleached ply is the weaker fiber, and it costs less because it has been through one fewer stage.

Yes, with one qualification. Four color process prints on kraft board, but printing inks are transparent, so every color lands over brown and comes off the press warmer than it was proofed. Warm colors hold well and cool colors move toward the board, with pale tints moving furthest. White cannot be printed without help: there is no white in the board to leave unprinted, so any white in the artwork needs an opaque white underbase laid down first, which is an extra plate and an extra ink. Where the artwork is a photograph or carries a precise brand color, white kraft is usually the right specification instead.

Yes, and unbleached fiber is the most robust of the four grades in a recycling stream. Kraft fibers are long and have never been through a bleaching stage, so they survive more repulping cycles before they are too short to form a sheet. Nothing in the board has to be separated out. The thing to watch on a kraft carton is the finish rather than the board: kraft is the grade most often specified with foil, and foil is a contaminant in proportion to the area it covers, which is minor on a wordmark and significant across a panel.

At the same caliper, yes, because it has not been bleached. The kraft process leaves cellulose fibers long and intact, and bleaching is a further chemical stage that shortens and weakens them. An unbleached kraft sheet therefore keeps its full fiber length, which is why it outperforms bleached virgin board and recycled board of the same thickness. That does not make it the stiffest board for a large carton: stiffness is governed by how far apart the outer plies of a sheet sit, which is what folding box board is built to exploit, so a large panel that has to resist bowing is usually an FBB question rather than a kraft one.