While aluminum foil can be used as a makeshift cookie sheet in some situations, it is not the most ideal or recommended choice for several reasons:
Heat distribution: Aluminum foil is not designed to distribute heat evenly like a proper cookie sheet or baking pan. It can cause uneven baking, leading to some parts of the cookies being undercooked while others are overcooked.
Durability: Aluminum foil is much thinner and less durable than dedicated baking sheets. It can tear or warp easily, especially if you're handling it with hot cookies or heavier items.
Safety concerns: Placing aluminum foil directly on the oven rack can pose safety risks. It might catch fire if it comes into contact with the heating elements or if hot grease or oil drips onto it.
Potential chemical reactions: Certain acidic ingredients in cookie recipes, such as lemon juice or vinegar, can react with aluminum foil and cause off-flavors or discoloration in the food.
Cleaning difficulties: Aluminum foil can become very messy when used as a baking surface, making cleanup more challenging than using a proper cookie sheet.
For these reasons, it's best to use a designated baking sheet or cookie sheet made from materials specifically designed for baking, such as stainless steel, aluminum, or non-stick coated pans. These baking sheets are designed to provide even heat distribution, are more durable, and are safer to use in the oven. If you don't have a proper baking sheet on hand, you can consider using parchment paper or a silicone baking mat as alternatives to line a regular baking sheet.