The brown crud on the inside of an oven window is typically a combination of grease, food particles, and other residues that accumulate over time from cooking. When food is baked or roasted in the oven, splatters and drips can occur, and some of these substances adhere to the window's surface. With repeated use, these deposits can become baked-on and turn brown, creating the "crud" you see.
The high temperatures inside the oven cause these substances to break down and form carbonized residues, making them more difficult to remove. The window's close proximity to the heat source can exacerbate the buildup.
Regular cleaning and maintenance can help prevent excessive accumulation of this brown crud. It's a good idea to wipe down the oven window after each use and address any spills or splatters promptly. If the crud has already formed, there are various cleaning methods and products available to help remove it, such as oven cleaners, baking soda paste, or vinegar solutions. Always consult the oven's user manual or manufacturer's instructions for specific cleaning recommendations for your particular oven model.