The Complete Handbook of Foil

Chapter 1: Understanding Foil

Foil is aluminum—not an alloy coating, not a laminate, but thin sheets of pure or nearly pure aluminum metal rolled to a thickness where it becomes flexible and cuttable by hand. To understand why foil works and where it fails, you must start with what aluminum actually does.

Aluminum is an excellent barrier to light, oxygen, and moisture. A sheet just a few thousandths of an inch thick stops oxygen from penetrating into the material beneath it—which is why foil wrapped around food prevents oxidation and flavor loss. It reflects heat efficiently, which is why professional kitchens use foil to shield the tops of baked goods from browning too fast. It conducts heat quickly and evenly, which is why you can use it as a makeshift pan liner or heat distributor. And it is chemically stable under most conditions, meaning it doesn't degrade quickly at room temperature or in ordinary kitchen environments.

But aluminum has a critical weakness: it reacts with acidic and salty foods. Tomato sauce, vinegar, citrus, and salt-heavy preparations will corrode aluminum foil—not catastrophically in a single use, but noticeably over hours or days. A tomato-based dish wrapped in foil and refrigerated for a week will show discoloration and degradation on the foil's surface. This is why you need to understand what foil is and how it behaves, not just reach for whatever is cheapest on the shelf.

Foil starts as ore—bauxite—which is refined into pure aluminum ingots. Those ingots are heated and