Smoke is normal. The first preheating is smoke, because there is a protective oil on the baking tube. There is also water vapor. Water vapor is the gaseous form of water. When water reaches its boiling point, it becomes water vapor.

At one standard atmosphere at sea level, the boiling point of water is 99.974°C or 212°F or 373.15°K. Water can also slowly evaporate into water vapor when it is below the boiling point. And at very low pressures (less than 0.006 atmospheres), ice sublimates directly into water vapor. Water vapor may contribute to the greenhouse effect and is a greenhouse gas.

Expanded Information:

p>Attention:

Oven Preheating:

Before baking any food, the oven needs to be preheated to the specified temperature in order to meet the baking time in the recipe. It takes time to preheat the oven, but if you don't do it enough, you may not reach the specified temperature. If you preheat the oven for too long, it may also affect the life of the oven.

Baking height:

With a small oven to bake something, basically just put the food in it, but the medium-sized oven usually contains the upper, middle and lower three heights to choose from, as long as the recipe does not specify the upper and lower fire temperatures, the baking tray can be placed in the middle;

If the upper fire temperature is high and the lower fire temperature is low, unless the upper and lower fire can be adjusted independently of the oven, otherwise, it is usually the upper and lower fire temperature can be adjusted. In this case, we usually add the upper and lower temperatures and divide by two, and then place the baking sheet on the upper level, but we still need to keep an eye on the surface to see if it's too burnt.

Baidu Encyclopedia - Oven

Baidu Encyclopedia - Water Vapor