Creating a two-tone or gradient patina on metal surfaces is a popular technique among artists and metalworkers to achieve visually striking finishes. Here are the most common methods used to produce these effects:

1. Chemical Layering: Applying different patina solutions in layers creates distinct color zones. Start with a base coat of one chemical (e.g., ferric nitrate for copper), then selectively apply a second solution (e.g., liver of sulfur) to blend or contrast tones.

2. Heat Application: Heating metal unevenly with a torch can produce gradient color shifts. Copper, for example, transitions from red to purple to blue as heat intensity varies.

3. Masking Techniques: Use tape or wax to block sections of the metal before applying patina solutions. Removing the mask reveals a sharp two-tone contrast.

4. Abrasion and Polishing: After applying a full patina, lightly sand or polish areas to expose underlying metal, creating a gradient or two-tone effect.

5. Differential Oxidation: Submerge metal partially in a patina solution or spray it at an angle to allow gradual color transitions.

These techniques can be combined for even more complex finishes. Always test on scrap metal first and seal the final patina with wax or lacquer to preserve the effect.