Fire-retardent and heat-resistant paints are used where there is danger of high heat or fire. Fire-retardent paints use an oil or oil resin base and have chemicals that cause it to blister and form an insulating barrier between the fire and the wall. The paint will burn but can be put out once the igniting fire is removed. Heat-resisting paints cover warm or hot surfaces such as ovens, boilers, or the cylender heads in aircraft engines. Alkyd-resin bases are used in paints for moderately hot surfaces while silicone-resin is used for higher temperatures. Very hot surfaces use a metallic pigment mixed with varnish to evaperate and leave only the pigment bonded to the surface.