The corals that look so pretty in a glass canister or on a mantel are really colonies of limestone polyps -- animals -- that attach to the ocean floor and build up hard skeletons over centuries into the formations we call reefs. In most places, it is illegal to disturb, damage or collect coral, as rising temperatures, ocean acidification and other threats are irreparably damaging the live reefs and their ecosystems. But some coral, the byproduct of legal dredging or drilling operations, can be harvested for sale. If you obtain a piece of coral legally and want to bleach it white, proceed cautiously: Limestone is fragile.
Coral bleaching that happens on a living reef produces ghostly white sculptures that lose their color due to stress. A vibrant reef turns pale when the corals are shocked or damaged and expel or lose the algae that is their food source and responsible for coral's normal color. The main cause of coral bleaching is warmer ocean temperatures due to climate change, but there are other stressors that contribute to whitening a reef.
- Storm run-off and pollution damages and bleaches near-shore corals.
- Prolonged high temperatures results in high solar radiance in shallow water, bleaching snorkel-accessible reefs.
- Shallow reefs are also vulnerable to extremely low tides that expose them to air.
- On rare occasions, the shock of unusually frigid temperatures during a cold spell can kill a coral reef.
Not all bleaching is a death sentence. If the stressor is mild and not prolonged, some corals can regenerate and survive. But bleaching makes the animals susceptible to disease and to any additional disruption, so recovery for a bleached reef is not simple and may not be possible.
Coral obtained legally is normally a dull off-white, yellowish or cement color, not the startling snow-white preferred for a dramatic sculptural display. To create white coral for jewelry or exhibition, you have to bleach it -- carefully. Limestone is calcium deposits and is fragile enough to disintegrate or dissolve, so proceed with caution when bleaching limestone coral.
- For coral recently submerged that may have live algae in its cells, place the pieces in a bucket of fresh water for three days. Coral is a saltwater animal and can't survive in fresh water, so the algae will die and release their grip on the limestone shells.
- Transfer the coral to another bucket with warm water and mild detergent. Clean off any dirt or sand with a sponge or a soft brush and rinse the coral.
- Then soak the coral in a bucket of plain water with a cup of household bleach and leave the bucket in the sun for a few hours. Err on the side of less time rather than more, depending on the fragility of the coral pieces you are bleaching.
- Finally, rinse the coral well in plain water and let it dry in the sun. Your coral should turn bright white and retain no briny smell -- suitable for permanent display.
Some people like to clear-coat the coral with non-yellowing lacquer for protection from dust. It's a good idea to display coral under glass because it is difficult to clean, and the bleaching process increases its fragility.