Updated: Nov 13, 2019
emp Essential baking mats (duh) are made of silicone, which is why they're so nonstick. As they heat up in the oven, the silicone molecules expand, absorbing the oils (and, consequently, some residual flavors and fragrances) of whatever you're cooking. After they cool down, it's very difficult to remove the oils from the mat; that's why they can become covered in an unpleasant, oily sheen.
Thankfully, we've got a few trusted cleaning methods that will have your silicone mat looking (and smelling) new:
Scrub your dirty silicone mat with a paste made of warm water and baking soda, let it sit for ten minutes, then rinse. Voila! Fresh and clean.
Soak your silicone mat in hot water dosed with the juice of a few lemons to give it a deep refresh.
Warm the silicone mat up in the oven first, so that the silicone molecules can expand and secrete the flavored, smelly oils. Then, while the mat is hot, immediately plunge it into a bath of warm water and white vinegar (or lemon juice). It should emerge smelling like new!