Method for making a layered glass article

A method for the manufacture of a solid layered article having a radial variation in composition, said article comprising at least two solid materials of differing composition forming multiple alternating spiral layers within at least a portion of the article which comprises the steps of:

• (a) providing a mixing zone for the shear mixing of a viscous liquid or plastic mass, which zone comprises a gap bounded by opposing surfaces, at least one of said surfaces having an axis of symmetry being rotatable with respect to the other about an axis of rotation coincident with said axis of symmetry;
• (b) providing in the mixing zone a composite charge of material which incorporates at least two discrete component charges of hardenable viscous liquid or plastic material, which differ in composition from each other, the component charges being geometrically distributed within the mixing zone such that the relative proportions of at least two of the materials varies as a function of lateral position across the gap bounded by the opposing surfaces;
• (c) rotating at least one of the opposing surfaces to generate shear stress across the gap and effect circumferential redistribution of material in the gap about the axis of rotation, sufficient to cause spiral layering of at least two of the component charges, and
• (d) removing and hardening the layered composite charge to form the solid layered article.