In an example of a 3D printing method, build material particles are applied to form a layer. Each build material particle includes a metal core and a metal oxide outer shell. The layer is patterned by selectively applying a reactive chemical on at least a portion of the layer to initiate a redox reaction with the metal oxide outer shells of the build material particles in contact with the reactive chemical, which reduces the metal oxide outer shells of the build material particles in contact with the reactive chemical and exposes the metal cores of the build material particles in contact with the reactive chemical. The patterned layer is exposed to rapid thermal processing to sinter the exposed metal cores to form a part layer. Any intact build material particles remain unsintered.