was used table ii______________________________________material percentage______________________________________sio sub 2 58 8 al sub 2 o sub 3 18 5 zno 12 5 li sub 2 o 5 3 na sub 2 o 3 3 sb sub 2 o sub 3 1 5 k sub 2 o 1 ______________________________________ the bracket 10 using the appropriate glass composition provides an inner amorphous portion 39 within the outer crystalline layer 38 the thin outer layer 38 does not substantially affect the clarity of the glass thus leaving the bracket substantially transparent or translucent the thin crystalline layer 38 has a thickness t in the range of 0 0005 to 0 005 inches 0 0127 0 127 mm preferably between about 0 001 to 0 004 inches 0 0254 0 102 mm the outer crystalline layer 38 provides a compressive force on the surface resulting in the substantial increase in the strength and fracture toughness of the bracket 10 the thickness t of layer 38 should not be too thick in relationship to the size of the product as this can cause too great a compressive force to be produced which can result in failure of the part the thickness t of layer 38 is preferably no greater than about 25 of the cross sectional thickness of the product at it 39 s thinnest point it is important in order to appropriately initiate the crystalline growth at the surface during the heat treatment process that the surface have an appropriate roughness applicants found that the surface roughness after the machining operation should be no less than about 4.