Tempered hardboard, or high-density fiberboard, is an engineered wood board that, unlike particleboard, is made with ground-up wood fibers rather than wood chips and held together with resins. Tempered hardboard is strengthened through a process of pressurizing and heating, and chemically treated with a thin layer of linseed oil or other drying agent used as a sealant.
Tempered hardboard is sturdier than other hardboard, meaning it's more rigid and yet bendable, with greater surface hardness and tensile strength. It is also more water-resistant than untempered hardboard and holds up better under extreme heat and weather conditions, making it the preferred choice for outdoor or exterior use, such as construction siding. Tempered hardboard is generally less expensive than untempered hardboard.
Not all tempered hardboard is manufactured the same, with disparities in factors like the oils or resins applied in the manufacturing process and the surface grades on each side of the board. Terms like "heat-tempered," "vat-tempered," "core-tempered" and "chest-tempered" only add to the confusion. Tempered hardboard is generally unsuitable for painting and is therefore a poor choice of hardboard for artists.
When choosing tempered hardboard for a project, determine which manufacturer's "version" of tempered hardboard best fits your needs and purchase all your tempered hardboard for any given project solely from that manufacturer. One commonality among all manufacturers, however, is that tempered hardboard is generally identified with one or two red stripes, while untempered hardboard is distinguished by a green stripe.