Serious Accident By Using The Proper Base
For Your Big Green Egg
by WileECoyote from EggheadForum.com
I immediately shut down the Egg and spent a few weeks researching all the related issues. I was surprised to find a number of stories and pictures of similar problems on the internet: burnt shelves, exploded tiles, cracked paving stones, etc. I even found a few people that had holes burnt completely through their outdoor decks and one unfortunate soul that had a serious house fire. Here are just a few of the many photos that you can find online:
This motivated me to do even more research and testing to be sure that my Egg would be safe, especially when cooking at high temperatures or when unattended for extended periods. My research uncovered lots of valuable and factual information from the internet, brick manufacturers, fireplace dealers, contractors, forestry and lumber services, forensic science reports, Underwriters Laboratory, and a key employee at the Big Green Egg corporate office whom I will not name due to liability concerns. After careful review of these materials and numerous tests with a digital laser thermometer, I concluded that the safest and proper base for my large Egg and table combination should be formed as follows:
I concluded that using a lesser base may introduce significant risk of the Egg, support base, and/or the surrounding materials cracking, exploding, charring, or burning, especially when using a thinner tile or not using the ceramic feet. This could cause a house fire or serious accident and the initial warning signs may not be visible during normal use of the Egg. The damage may happen gradually over time so I highly recommend that you periodically check under your Egg and correct any problems that may exist before it is too late.
Exterior Egg Temperatures: The outside of the Egg gets very hot while in use. Exterior temperatures can range from 100-400+° F depending on the internal temperature and duration of the cook. Secondary factors include the outdoor temperature, nature of any surrounding materials, and the degree of air flow around the Egg. The bottom is one of the hottest places of the Egg because it is closest to the red hot coals. The fire box is open on the bottom and the cast iron fire grate is thin so there isn’t nearly as much material between the hot coals and the base as there is on the sides and top of the Egg.
Table Materials: Most Egg tables are made of wood because it is relatively cheap, easy to work with, looks good, holds up well, and is readily available. Most table designs place a wooden shelf or wooden support rails directly under the Egg. These wooden surfaces are almost always stained and covered with urethane or a similar finish. This is a concern for reasons covered in the following sections.
Flammability: Wood burns when it is exposed to excessive heat. The starting temperature for the combustion process varies based on many factors. Some of the most significant factors include: moisture content, density, size, finish, amount of heat, and duration of exposure. Moisture must be evaporated from wood before it will burn, similar to the way that the collagen inside a pork butt must be broken down into gelatin before the internal meat temperature will rise to the final peak. Wood can dry out rapidly when exposed to high heat or gradually over time when exposed to low heat, thus most wood that is in proximity to the Egg will become more flammable over time as it ages. Dense wood requires more heat to ignite than light wood, so the large pieces of cedar, cypress, or pine found in most Egg tables will tend to be more flammable than other woods. Wood generally ignites at a much lower temperature after it has been stained or treated, unless it has been treated with special fireproofing compounds, so by finishing the Egg tables we are making them even more flammable than normal.
Combustion: Numerous scientific studies and forensic tests have determined that most woods used in residential construction will begin the combustion process at or near 170° F under average or typical conditions. By using lighter wood, staining it, and drying it out over time we are actually lowering the combustion point by a significant amount. There is no set temperature or magic formula for determining the actual combustion point, however, due to the variables mentioned above in addition to many other variables which are too technical for the scope of this article. There have been numerous documented cases of wood combustion starting as low as 90-130° F under extreme conditions so it is safe to say that we want to minimize the amount of heat that is transferred to the wood surfaces under and around the Egg.
Insulation: Numerous materials are used as insulators to minimize the transfer of heat between objects. Air is one of the best insulators which is why the BGE ceramic feet work so well: they keep a cushion of air under the base of the Egg where the extra protection is needed the most. This air gap is critical because any other material will not insulate nearly as well and thus we will be transferring much more heat directly to the support structure if we avoid using the feet. The base of the Egg gets extremely hot as mentioned previously so the ceramic feet alone are not adequate protection when the supporting material is wood or a similarly flammable substance.
Base Material: BGE has recommended a concrete paving stone under the Egg and many people have used marble, granite, ceramic tile, bricks, metal, or other materials. Contrary to popular belief, most stones, concrete, tile, and brick are not good insulators. Their dense, heavy composition allows them to absorb a lot of heat instead of reflecting it and thus they initially seem to be good insulators when in reality they are just soaking up the heat and storing it internally. When exposed to heat for extended periods of time they will eventually become almost as hot as the heat source itself and then radiate this heat outward for a long period of time until they are cooled down by circulating air or some other method. Most metals are not a good insulator either because they are good conductors and will rapidly transfer heat to the surrounding surfaces. Concrete and ceramic tiles are cheap and readily available but they are not ideal for high-heat applications because they are porous and will absorb moisture; which in turn causes them to crack or explode when they are repeatedly heated and cooled. Using a cracked support base could lead to a cracked or damaged Egg due to uneven support and the force of the exploding tile, and it is also a fire hazard since extreme heat would radiate down through the cracks and be exposed directly to the wooden support structure underneath the base. This is why I concluded that firebrick is the best choice for my application. It is specifically made to be very dense and non-porous so that it does not absorb or retain a significant amount of moisture; thus it will rarely crack or explode unless subjected to other forces. The firebrick will still absorb and retain heat but it should be able to dissipate it safely as long as an air gap is left between the support rails and the area around the firebrick is kept open for air to circulate.
Regardless of which base you choose for your Egg, to be safe I highly recommend that you periodically check under your Egg for any signs of a cracked base or charred wood. Replace or redesign your support structure as needed at the first signs of trouble. Remember that you probably won’t see any smoke or smell the wood burning while the Egg is in use since the charcoal and wood chips will mask it and the combustion process can happen very slowly over time. Life is precious so please don’t take any shortcuts or chances. You might even want to invest in a digital laser thermometer to take your own measurements. I found mine on sale for only $30 at HarborFreight.com but they are also sold in most hardware stores. This was a bargain compared to the peace of mind I get by knowing that my Egg is safe, and I still use the thermometer periodically to measure grate, pit, skillet, and interior dome temps while cooking. (See link to Harbor Freight below.)