Hammerscale, also written hammer scale, is a flaky or spheroidal byproduct of the iron forging process (for modern equivalent, see mill scale). Hammerscale is almost universally recovered from archaeological excavations in areas where iron ore was refined and forged. Hammerscale’s magnetic character also aids in its recovery and in mapping larger features by means of magnetic susceptibility surveys. Hammerscale can provide vital information about an archeological site such as the function of the feature.

Hammerscale appears in two forms: flakes and hollow spheroids. The flakes can vary greatly in appearance and size; however, their color ranges from a dark black to a lustrous blue or grey and their thickness from one to five millimeters. Like the flakes, the spheroids are also quite small but their size can vary. Their color tends to range from grey to a dark black or blue.

The chemical composition of hammerscale is disputed and varies greatly. Most sources agree that hammerscale is composed of some form of iron oxide. Magnetite is a generally accepted form, giving hammerscale its notable magnetic character. However, hammerscale’s chemical composition can change depending on from which stage in the iron purification process it derives; for flakes and spheroids from early stages, the composition will be largely mixed whereas flakes from late stages will be purer iron oxide forms. Ultimately, some still argue that the chemical components of hammerscale besides iron vary widely beyond ionized oxygen to form a metal oxide.

Flake hammerscale forms due to the rapid oxidation of hot iron in air. A heated piece of iron will develop an external layer of iron oxide which then may separate from the original piece due to a hammer strike or differential thermal contraction.

Flake hammerscale is produced in large quantities during multiple stages of the smithing process. To create the pure iron necessary for forging, a smith must first purify the iron ore. The smelting of ore creates a "bloom", a porous mixture of slag and metal. The smith then repeatedly heats and hammers the bloom to remove impurities. This technique creates hammerscale of varying composition. As the purification process continues, the hammerscale’s iron content increases.

Additional hammerscale is produced during the forging of the pure iron from the heating and hammering necessary to shape the piece. The scale produced at this stage is characterized by its blue-black color and tends to be slimmer and darker due to its high iron oxide content.

Archeologists believe that spheroidal hammerscale is produced primarily during the process known as fire welding. Also known as forge welding, this technique is used to connect two pieces of metal by heating them to a high temperature and forcing them together with a hammer or other tool. For this method to be successful, the surface of each piece of metal must be molten. As the smith hammers the pieces together, some metal is forced out from between them, often in the form of a molten jet which cools in the air to form spheroidal hammer scale.

It is also possible for spheroidal hammerscale to form during the purification of bloom steel. Iron oxide can combine with silica, from the raw ore, to form slag. As the bloom is forged and refined, the molten slag is driven out. Once the slag cools, spheroidal hammerscale is formed.^{[citation needed]}