Patina Development: Tidal Memory

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November 2024

The bronze casting for Tidal Memory came out of the foundry three weeks ago. Clean bronze has a certain appeal, but this piece needed surface treatment to suggest weathering—specifically the greens and browns you see on objects that have spent time in saltwater.

Initial Tests

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Started with small test plates to work out the color range before applying anything to the actual piece. The goal was sea-glass green with variations toward oxidized brown in the recessed areas.

Basic copper sulfate solution gave decent green, but too uniform. Real weathering isn’t even—it pools in low spots, gets rubbed away on high points. Needed a process that would create that naturally.

Layering Approach

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Rather than trying to get the right color in one application, decided to build up layers:

1. First pass: Diluted copper sulfate, applied with natural sea sponge. Let it oxidize overnight.
2. Second pass: Salt spray (actual seawater, not mixed salt solution). This created the streaking that happens when salt water runs down vertical surfaces.
3. Wait 48 hours.
4. Third pass: Liver of sulfur in selective areas—this darkened the recesses without affecting the green zones much.
5. Final pass: More salt spray, very light.

Between each layer, I’d rinse with distilled water and let it dry completely. The drying time matters—trying to rush it led to blotchy results on the test plates.

Selective Polishing

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Once the patina had developed for about two weeks (checking it daily, sometimes adding more salt spray), certain areas got selective polishing with fine bronze wool. This removed the patina entirely in spots where water would theoretically wear it away—the highest curves and edges.

The contrast between polished bronze and oxidized surface is what makes it read as weathered rather than just colored.

Sealing

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Commercial patina sealers tend to make bronze look like it’s coated in plastic, which defeats the whole purpose. Instead, using a thin wax—Renaissance Wax, applied warm and buffed after cooling. It protects the surface without looking like protection.

The wax will need reapplication every year or two if the piece stays indoors. For outdoor installation, I’d skip the wax entirely and let real weathering continue the process the patination started.

Variables

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This process worked for this particular piece and these specific conditions (humidity in Thailand averages 70%, temperature was 28-32°C during treatment). Different climates would need adjustments.

Also worth noting: bronze composition matters. This was a silicon bronze alloy, which responds differently to patination than traditional bronze would. The silicon content seems to make the greens more stable.

Time Investment

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Total elapsed time: three weeks from clean casting to sealed piece, though actual working time was maybe six hours spread across those weeks. Most of the process is waiting for chemistry to happen.

You could theoretically rush it with heat and stronger solutions, but then it looks rushed. The slow oxidation creates depth that quick patination doesn’t have.

The finished piece is in the Works section. The surface will continue developing subtle changes over time, even indoors—that’s part of working with living materials like bronze.