Yeah you gotta watch out with anything you heat with. The hot rocks are definitely a "No-Go." But even the pads that go under the tank get pretty hot. I experimented when I first set up my tank. I place the heat pad under the tank, plugged it in, gave it about an hour to warm up and then measured the temp with a hand held infrared thermometer. It was about 110 degrees. Way too Hot!!!!
So I got some old newspapers, folded them, stacked them on top of the heat pad, and then gave them time to heat up. Even with almost an inch of newspaper on top of the heat pad the temp still measured 94 degrees. That's good for a boa but too hot still for a corn snake.
I ended up buying a Rheostat that you plug the heater into. You can vary the temp by turning a dial. If I turn the dial all the way up, I still get about 105 degrees. But if i turn it all the way down I get about 89 degrees. Yup, all the way down and it's still 89 degrees! That's better than before, but it's better to be around 83 to 87 for Corn snakes.
So then I put in the furnishings. I have the "Repti-Mat" covering the floor of the cage, and a few hides, water dish....etc....... These coverings and furnishings absorb some of the heat, and when I took another reading, it was 84 degrees over the area where the heat pad was. I actually turned the rheostat up just a hair and got the temp to where it reads 85 now all the time. I put some duct tape over the knob so it would not get moved.
Every time I do a cage cleaning, I check the temp. It now stays at 85 all the time.
2 things I learned from this - 1). You absolutely need one of those infrared thermometers. They are great! Just point and you get an instant reading. and 2). You absolutely need a rheostat or thermostat to control your heat. The heat pads are just too hot by themselves, and you can't just place your hand on it and go by the way it feels to you. Your Body temp is 98.6, what feels good to you or me doesn't necessarily feel good to an animal.