Woodwork exposed to fire temperatures will char, whether or not it is coated, so that no paint can be regarded as fire-proof. The most that can be expected of any paint-type coating is to stop or retard the spread of flame along the surface; the extent to which it will succeed in doing so will depend to some degree on the thickness and nature of the paint.
Both oil and water paints can be made more resistant to fire by the addition of borax, magnesium phosphate, and similar materials. The latter are not, however, ingredients of ordinary paints and consequently tend to decrease the durability of the paint film on exposure. Furthermore, they are water-soluble and are inclined to leach out under the weather, with a consequent loss of fire-retarding efficiency. The addition of chlorinated rubber increases the resistance of paint coatings to fire. Sodium silicate is also used for the same purpose: the protection given by this substance is mainly due to its property of intumescence, 1.e. the swelling of the film on exposure to heat to a frothy mass which insulates the wood against heat. The chief weakness of such coatings is their instability.
Up to the present it would appear that a paint which is both fire-retard-ant and reasonably durable, and capable of giving a full range of decorative effects, has not yet been produced, but there are indications that there may be developments in this respect. Research being carried out with certain types of synthetic resins containing ammonium phosphate are said to yield promising results.
It may be observed that although they are seldom sold specifically for the purpose, most water paints and distempers have good fire-retarding properties.