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by Charles Weber, MS

This article presents an effective and inexpensive way of fire proofing wooden buildings using sodium silicate solution (water glass). There is also a technique of using aluminum foil for wood and Portland cement additives for protecting asphalt roof shingles against fire and ultraviolet light discussed.

If wood is coated with sodium silicate solution the wood will no longer support combustion even after it dries. This procedure is satisfactory for any wood not rained on such as basement beams, attic beams, wall studs, or unpainted wooden walls (in warehouses for instance). The hardness and strength of the wood is increased. With a protective coat it would probably be useful outside also. Not only is the material very inexpensive, but so is the labor. It can be sprayed on quickly with an ordinary garden insecticide sprayer, as well as all the usual tools for applying paint. It should also be practical to dip wood.

I can warmly recommend coating all undecorated wood in earthquake fault zones, especially buildings served by gas or near such buildings and in areas surrounded by dry brush

I suspect that by coating a wooden plywood fence it would make an inexpensive fire break in a forest as well. If the fence were placed in a swath of short grass and a wooden pipe sprinkler system installed, there should be few fires get past it, assuming no high wind. It is nutty to deliberately set fire to woodland to remove underbrush as is now often done. That brush should be used as a farm soil amendment or at the least, used to generate electricity. It could be harvested usually less expensively and more safely than coal. At the same time there would be less fire men and coal miner’s widows. Our forests are extremely valuable as a source of wood, for recreation, and to help cool the atmosphere (see this site). They are very important in preventing soil loss where rainfall is adequate. To cavalierly allow them to burn because we are too lazy or too stingy to protect them is idiotic.

I have used this procedure often and have never had any problem other than over spray staining some varnishes. I have even sprayed the diluted silicate into my eyes accidentally, with no obvious harm other than the sting one gets from soap in the eyes. I have not had full strength sodium silicate in my eyes, so use caution. Sodium silicate is caustic and will etch glass eventually, so it may be that you should not wear expensive prescription glasses when working with it. I suspect there would be no problem if it is wiped off in a reasonable time, but I have no experience. Sodium silicate is not toxic, although it is possible that breathing the spray could be conceivably harmful. For this reason, if it is sprayed, a garden manual sprayer is probably the best tool since the spray is coarse and works very well. An airless sprayer with a mask is the fastest way for large jobs. Thousands of square feet can be covered per day for sure with no danger of corrosion to the equipment.

The wood changes appearance to a gray mottled efflorescence when the silica precipitates out between the fibers and on the surface. Instead of being a tinderbox your structure will become petrified. It is extremely effective. A direct flame will char the wood but it will not support combustion.

Sodium silicate can be stored in almost any kind of container including steel, this last since silicate has rust inhibiting characteristics. Concentrated sodium silicate is said to corrode aluminum. It must be stored out of contact with air since carbon dioxide in air slowly converts it into a much less useful mixture of silica and sodium carbonate (sal soda). If the surface must be bright to reflect light or appearance counts, it is possible to white wash it with lime. I have only limited time experience with this last procedure. The coating lacks toughness of course. I have reason to believe that acrylic automobile enamel (actually a lacquer) is compatible with dry sodium silicate. This may prove to be a way to use this procedure outside and may work to maintain the wood color, but I have no experience with this last. Automobile enamel is very expensive. However diluting it with solvent could possibly make a less expensive primer coat out of it for less expensive latex paint. Be very careful with that enamel in closed spaces because I have good reason to believe the fumes can trigger an attack of gout. I have not tried acrylic concrete paint (also really a lacquer), but I suspect it would work. This combined technique may not be less expensive than fireproof paint, but it should be more resistant to ultraviolet and probably more fire proof. Acrylic latex paint is incompatible with fresh sodium silicate but aged surfaces may prove to be OK. Also it is possible that a wood stain might work also for appearance.

Another way of making sodium silicate impervious to rain is to spray a thin layer of vaporized silicon monoxide. In the presence of air silicon monoxide oxidizes to silicon dioxide (silica) I do not know of the paint compatibilities of the resulting coating. Compatibility should be good though.

It is said by PQ Corporation that multi valence positive ions such as aluminum in aluminum sulfate solution will set silicates into an insoluble film. This could conceivably make silicate films suitable for out door use, especially if it proves compatible with a paint. Just make very sure not to use anything containing fluoride, since fluoride is very poisonous, more so than lead and almost as much so as arsenic (see this site).

Silicates have been used in combination with styrene-butadiene latex for exceptional protection of clay-filled paper board and sulfite-lined cylinder board against turpentine, cotton seed oil, and lard oil. A silicate-latex combination also protects against transmission of water vapor. So it is possible that such a paint composition would be compatible with a sodium silicate coating. Tom Rawlings has reported to me that shellac coats silicate successfully. If so, it should make a good primer for other kinds of paints. I suspect that silicated wood could be stained successfully and is worth trying.

Potassium silicate is similar to sodium silicate. Lithium silicate creates a less soluble film, especially if heated gradually up to 425 degrees F. However, it swells much less than sodium silicate under a flame [Veinot].

Silicate films can be made opaque by the use of titanium dioxide or aluminum pigments. Fillers such as clay are used for semi-opaque films. Alkali resistant pigments are necessary for use with silicate vehicles. The following are suggested: white - titanium dioxide; red - lime-free iron oxide; blue - ultramarine; green - chrome oxide; yellow - ochre; brown - umbers or siennas; black - grease-free carbon black.

Not only are the pores of the wood blocked from feeding combustible gases to any flame to a considerable extent, but a ceramic foam builds up on the surface through release of the carbon dioxide from the resulting sodium bicarbonate that has formed. The latter tends to insulate the wood from the flame's heat. The net result is that the wood can not support combustion.

There is a fringe benefit to silicates. Such treatment probably makes the wood more resistant to termites. If termites are a problem, by mixing some borate (borax) with the silicates, it should make the wood even more resistant to termites (isoptera), and borate itself tends to make wood fireproof. Thus two goals would be reached with a single application of nonpoisonous material. If you come across any ingenious techniques for adapting sodium silicate to fire proofing or painting it, please let me know. You can easily see how well it works by coating some scrap lumber, letting it dry, and hitting it with a propane torch.

SOURCES OF SODIUM SILICATE, see end of article.


Another good way of fire proofing wood is to glue aluminum foil to it with solvated, fibrated asphalt. This prevents the wood from burning when assailed by a flame for a long, long time. Aluminum foil is fragile so it is advantageous to protect it with mobile home roof acrylic if it is walked on.

It is a procedure which is especially useful on a wooden roof because it has the additional attributes of:

1. The roof is water and rot proofed. The water proofing can not be depended on so any joints in the plywood must be sealed well.
2. The asphalt is protected from ultraviolet degradation and so has an indefinite life.
3. The materials are very inexpensive, and so usually is the labor fairly so.

It is possible that combining the above two procedures would be especially advantageous. I have never tested this combined procedure so if you try it, do it on a small test scale first. One respondent has reported that sodium silicate is a good adhesive for aluminum. It does seem plausible that sodium silicate would bond to aluminum since aluminum has a thin coat of aluminum oxide on the surface. I assume it would be a more brittle bonding agent than asphalt. It does not look to me as if there should be any serious problems, though, and is worth trying. If it had been in place at Chenobryl, that disaster probably would never have happened. Keep in mind, though, while aluminum is an excellent reflector of light, it is not very good at reflecting infrared radiation and it will not do so inside a wall. Also it will not transmit water vapor so will not breath.


Another fire proofing procedure works well on asphalt shingle roofs. This involves coating the roof with about one eighth inch of Portland cement made up with butadiene-styrene latex formulated for this purpose. Such a coating is much more resilient than Portland cement, almost as resilient as box wood. It is very adherent to many surfaces. One of its chief advantages, however, is that it dry cures. Unfortunately it is no longer available in retail quantities because of Dow Chemical Company's fear of product liability and is only available to approved bridge contractors, from whom you can sometimes get it. Acrylic latex additive would probably work also and may be less damaged by ultraviolet. It is available as the Probond and Bonsal trade names.

Such a coating is quite fire resistant. It is comparable to the aluminum foil mentioned above. It can be brushed on very easily. It is slightly soluble in water, however, like all Portland cement. Therefore an eighth of an inch will erode off in 7 or 8 years in a climate like the northeast USA. For this reason it is desirable to paint it with an acrylic latex paint or a concrete paint made mold resistant with a poison, which should be capable of inhibiting algae also. An additional advantage of this step is that it enables any color. This procedure changes the texture of the shingles. Some may prefer the original texture. It could probably be reestablished with the correct knife and/or trowel while uncured, although this would be labor intensive. However there is no substitute for a fire resistant roof, especially in a dry forest.

An additional advantage of this procedure is that the shingles are protected from ultra violet degradation. I do not know if this keeps the shingles intact indefinitely, but I suspect so. It is a good way to salvage a roof which is starting to deteriorate. Replacing a roof every 10 or 20 years can be noticeably expensive.


Fire insurance was a good invention as a financial safety net. However, there is no really good substitute for not having the fire in the first place. Money can be the least of what can be lost. If you are sitting out in an icy gutter holding some money, it can be a big help, but it is no substitute for emotional peace of mind, priceless heir looms and antiques, disruption in one's life, laboriously gathered photos, data and papers, or most important of all, family members.

You may see an article describing treatment of wood with various silicon compounds here. and one describing miscellaneous other compounds here. Don’t use cancer causing asbestos, though.

The author, Charles Weber, has an MS degree in soil science at Rutgers University. He may be reached by this email; isoptera at –or—by telephone; 828 692 5816 (USA)


In some parts of the world it may be too difficult to acquire sodium silicate. This article describes how it is made. Wood ash may be possible to make potassium silicate. This organization gives sources of chemicals, etc.

The sources below are old and some may be obsolete now. However, you should be able to find them with an internet search or Thomas Register. Searching on the internet may possibly be assisted by the following synonyms; water glass; soluble glass; silicate of soda; sodium orthosilicate; silicic acid sodium salt; sodium silicate glass, Na4O4Si.

Post Apple Scientific provides. sodium silicate in 500 milliliter, one gallon, 5 gallon, and 55 gallon containers, from 8893 Gulf Road, North East, PA , USA, 16428. Phone 1-814-434-8801.

Sodium silicate may be obtained in one and 5 gallon containers from:

Sodium silicate is available in 5 gallon cans from; Industrial Chemical and Scientific Co., 11722 Charles St., Houston TX 77041, phone = 715 455 8776 or 800 332 4047.

Ashland Distributors in 55 gallon fiber drums , for about $350 phone 800 522 1409.

PQ Corporation; (available also as a soluble powder as well as a solution) Be sure to acquire the easiest to dissolve powder if you go this way.
CORPORATE HEADQUARTERS PO Box 840 Valley Forge, PA 19482-0840, Phone: 800-944-7411
IN CANADA National Silicates, Phone: 416-255-7771
IN MEXICO Silicates y Derivados, S.A., Phone: 011-52-55-5227-6801
IN EUROPE PQ Europe, Phone: 31-33-450-9030
IN AUSTRALIA PQ Australia Pty. Ltd., Phone: 61-3-9708-9200
IN TAIWAN PQ Silicates Ltd., Phone: 886-2-2383-0515

Ruger Chemical Co. as 20 liters for $43 (but $150 minimum), phone 973 926 0331.

PQ Chemical Co.,

Dooner and Smith in NJ, USA, sells 5 gallons, but only over the counter.

Thomas Register lists some manufacturers who may possibly furnish dealers or sell retail as follows:
Chemical Services, West Chester PA.
North Metal & Chemical 888 372 6177
Occidental Chemical Corp. Dallas, TX.
C.E.D. Process Industries 330 666 2400.
GFS Chemicals, Inc (high purity) 800 394 5501.
Captree Group, Inc (laboratory supplies) 631 841 0200.
Alfa Chemical Corp, Kings point NY.
Mil-Spec Industries Corp., Roslyn Hieghts, NY

There is said to be a very fire resistant coating, although expensive, with which I have no personal experience and borates also fire and termite proof wood.

You may see innovative ways to repair macadam tennis courts and macadam parking lots at this site.


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Veinot DE Languille DT Nguyen GN Bernt JO 1990 Fire protective coatings based on ternary Na/K/Li silicate system. Journal of the Canadian Ceramic Society. 50; 32-36.

Email: to Charles Weber = isoptera at or 828 692 5816 (USA)

Updated June 2012