Different compositions for treating wood against fire are known.  They may contain components such as fine-grained solid material which have fire retardant properties, or intumescent compounds which form an insulating char layer on the coated article in high temperature, or compounds which are impregnated into wood to form for example steam in the case of fire.  Many compositions contain compounds which are considered toxic or harmful. 
WO91/00327 discloses a water soluble fire retardant composition for impregnation of solid wood products and other cellulosic materials and comprising from about 40% to about 70% of boric acid, at least one nitrogen-containing synergist material which is soluble in water at a solution temperature of from over 0° C. to under 100° C. in an amount which, in combination with boric acid, is effective to enhance the fire retardancy of wood products impregnated therewith as compared to that of the synergist material and boric acid alone, and a combined total of no more than about 15% of materials having an acid strength greater than boric acid, such composition, when introduced into wood products, having substantially no effect on hydrolysis of the wood cellulose and corrosion of metals in contact with the wood product. 
U.S. Pat.  No.  6,911,070 discloses a process for obtaining a liquid colloidal soluble fire and flame retardant and inhibiting mixture for natural and porous synthetic substrates comprising the steps of: a) mixing, as first step, 3-8 percent derivatized cellulose in water to obtain a colloidal solvent; and, b) adding 15-40 percent of a charring agent, and of a blowing agent to the colloidal solvent, to obtain an intumescent base; and, c) allowing insoluble components of the intumescent base to sediment and collecting a supernatant; and, d) dissolving a source of phosphoric acid in the supernatant, to obtain a flame-quenching mixture. 
WO2008/150157 discloses a fire-retardant composition consisting essentially of water-soluble flame-extinguishing agent, an intumescent agent and a complexing agent, and optionally a charring agent.  Phosphate-containing agent, such as ammonium hydrogen phosphate, is used as the flame-extinguishing agent. 
There is still need for fire retardant compositions which are economically and environmentally acceptable.  The use of large amounts harmful compounds, such as phosphor or boron, should be avoided, as well as excess release of undesired gases, such as carbon dioxide.  Further, such compositions should be stable for a long period of time and they should be applicable to several types of materials, such as wood.