Organic Chemist here: Those test-kits turn colors with everything! Based on a dilute aqueous solution of cobalt thiocyanate, they turn the "it's Lead!" color with laundry-detergent, ink on dollar bills, flour, etc. Here's a very simple test:

Take a paint-chip. Hold it with tongs/tweezers. Hold it in a flame. Now watch the color of the flame. Lead will color the flame a bright crimson-red. Avoid breathing your experiment regardless of content.

It was good to know this. One time we were stuck at a cabin on Cape Cod. It was about 20F out. The owner's son hauled in some wood from under the house and started a fire in the fireplace. I noticed the crimson color of the flames and got everyone out. We were better off going to a motel, rather than spend another second in that gas-chamber.

Beware of burning treated wood.