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The Slot Fire
or The Physicist's Fire

by Lawrence Cranberg, Ph.D.
Copyright 1995

Summary Copyright 2005 by
Robert Wayne Atkins, P.E.
All Rights Reserved.
The following summary is for fair use and educational purposes only.

A review of Dr. Cranberg's book is below. First, however, I would like to make a few comments about fires and the traditional wood burning fireplace.

After you get over the novelty of having a nice wood fire burning in the fireplace inside your home, two things become obvious to you:

1. Other than toasting marshmallows or cooking hotdogs, the primary purpose of the fire is to provide heat for your home.

2. When the fire has completely burned itself out, you are left with nothing but a pile of ashes in the bottom of the fireplace. That pile of ashes looks EXACTLY the same regardless of whether your are using UNsplit logs or split firewood.

When the significance of point number 2 sinks in, you quickly realize that unless there is a good reason to split your firewood, then you are simply investing extra labor that yields no positive return. (Unless you need the exercise and you have absolutely no other manual chores to do.) Therefore, any system that can provide a good fire using UNSPLIT logs is a real time and energy saver.

If you purchase your firewood, and only split firewood is available, then it doesn't matter. But you might ask your firewood supplier if there is a price difference between split and unsplit logs. If there is a big price difference, then the next question would be the average size of the unsplit logs. If the average size is what you can burn in the Texas Fireframe, then you could save some money by purchasing the unsplit logs instead.

On the other hand, if you are cutting your own firewood then may I please offer a suggestion. After you have the tree SAFELY on the ground, leave it ALONE for at least six to eight weeks. Most people want to start cutting on the tree immediately but that is not the best course of action. If you will leave ALL the limbs on the tree, and the top on the tree, then they will suck the moisture (sap) to the tips of the branches and help to quickly dry out your recently downed tree. After waiting about two months you will discover that the wood is reasonably dry and it doesn't weigh very much, because most of the moisture in the wood is now gone. However, if you start cutting the tree into firewood size lengths immediately after you cut the tree down, then the moisture will be trapped in each piece of wood and it will take a LOT longer for those pieces to dry out in the sun.

I purchased a Texas Fireframe Grate in 1999 and I have used it in my home every winter since then. The grate uses NO electricity and it will fit into any standard size fireplace. And it really does significantly increase the amount of heat that is actually transmitted into my home from the burning logs.

There are four major advantages to the Texas Fireframe Grate, in my opinion:

1. You get a LOT more heat into your home from the burning logs and it doesn't require any electricity or a fan.

2. You can use UNSPLIT logs. This is a major factor for me. Cutting down a tree and then cutting it into firewood lengths is only part of the job. If you then have to split each of the logs, you expend a LOT more effort. And if you are trying to conserve your energy, then splitting logs is not the way you want to spend your time.

3. The UNSPLIT logs can easily be lit with a few sheets of newspaper rolled up and placed in the center cavity between the logs. Until you actually do it, you probably will not believe that is possible. With a regular grate, you need newspaper and LOTS of small tiny sticks or kindling to start the fire. Then you gradually add slightly bigger sticks until the fire is burning well enough that you can start putting your split firewood on the fire. None of the kindling is necessary with the Texas Fireframe. And a really good fire is heating your home in about 15 minutes instead of 30 minutes (or longer).

4. The risk of a chimney fire is very close to zero with a Texas Fireframe grate. If you are forced to use a wood burning fireplace as a primary source of heat for an extended period of time, it would be nice to feel safe that your home isn't going to burn down as a result of a chimney fire.

The major disadvantage of the Texas Fireframe is its cost. However, the Texas Fireframe is made of very sturdy material, as opposed to the normal fireplace grates you typically find in the hardware stores. Over the years, I have burned up and replaced several of those cast iron grates. After five years of using my Texas Fireframe each winter, I can't see any damage anywhere on the entire unit. My guess is that it will easily last another 15 or 20 years, or maybe longer. Therefore, if you consider the replacement cost of a cast iron grate every few years, the cost of the Texas Fireframe grate is probably about the same as several cast iron grates over a 20 to 25 year time period. And when you factor in the difference in price (or labor savings) from UNsplit logs, then the Texas Fireframe is clearly a lower cost option over the long run.

I can personally verify that the statements made by Dr. Cranberg regarding the operation and efficiency of the Texas Fireframe Grate are not overstated. A very comfortable fire can be started on the Texas Fireframe Grate using UNSPLIT logs and a few sheets of newspaper. The resulting fire burns well and a significant amount of the heat from the fire enters the room and does not disappear up the chimney.

Normally I use the approximate size logs that are recommended for the grate. I always place the biggest log I have available on the rear of the grate. However, as I burn through my firewood each winter, I sometimes use two medium logs on top (instead of one), and three smaller logs on the bottom (instead of two). Even with these minor changes, the grate still provides a significant amount of heat for my home. I have occasionally used split firewood, and that also does exceptionally well on the grate.

As a very satisfied customer I am providing the following excerpts from Dr. Cranberg's book about the Texas Fireframe Grate.

The Slot Fire
The Physicist's Fire
printed in TIME Magazine
December 22, 1975

Cranberg built the "Texas Fireframe," a spindly metal contraption that looks like a standard fireplace grate with two taller upright at the front corners fitted with adjustable metal arms that extend into the fireplace. To use it, he places a large log toward the rear of the grate, two smaller ones toward the front, and a fourth log, slightly smaller than the first, on the adjustable arms (see diagram). He then lowers the arms until the top log just touches the surface of the large one at the rear. This creates a cavity that opens into the room - a sort of wooden furnace that contains the fire and prevents much of the heat from immediately escaping up the chimney.

One product of this arrangement is a hot, even, slow-burning fire: about 30% of the heat generated inside the slot eventually streams out into the room. There is another bonus: it is easy to light. A conventional fire requires a pile of kindling, a few balls of crumpled newspaper, and frequently several matches before it will catch. Often it burns for half an hour or more before it starts dropping coals and throwing off substantial heat. Because his arrangement traps heat so well, Cranberg can light even damp wood with only a few sheets of newspaper, placed directly in the cavity, and have a hot fire in 15 minutes.

The Slot Fire
by Lawrence Cranberg, Ph.D.

Chapter Four

First, a simple question: How good is wood as a fuel? How much heat is produced by burning own pound of wood? The answer is 8,000 BTU's, or the heat necessary to raise the temperature of 86 pounds of water 100 Degrees F.

It takes 6 pounds of air to burn 1 pound of wood, since air is only 20% oxygen. To heat the combustion air from the outside, say at freezing (32 degrees F) to the warm inside (say 72 F) - a 40 degree difference, you merely multiply the 40-degree difference by the 6 pounds of air - and then the happy part that very few people know: It is 5 times easier to heat one pound of air than one pound of water. So you only need 40 x 6 x 1/5 BTU's to heat the air, or 48 BTU's. That is about 1/200th the heat gain from burning one pound of wood. Only if you drag in about 200 times or more air than is needed for burning one pound of wood is wood-burning a break-even or losing proposition.

Next question: How much excess air is drawn into a fireplace. ... With the best arrangement possible from every point of view (The Physicist's Fire), the excess air is not 200 times but about 2 times what is required for combustion of the wood, and the fireplace is out front by a factor of 100 - a clear winner. (Chapter 4, Page 4.1)

Chapter Seven

Texas Fireframe Grate A large log (six inches in diameter or more - the larger the better) is placed on the back of the grate. Adjacent to it is a smaller log - three inches in diameter at most, with a similar one next to it. An upper log suspended by the distinctive height-adjustable arms of the Texas Fireframe grate contacts the back log and makes a snug cavity that opens to the room. Rolling a few sheets of newspaper that are inserted into the cavity, and a match are all that is needed to start a fire that burns steadily with exceptional intensity and left-to-right uniformity with no attention for about two hours.

The flames of the fire are thick, brilliant yellow-orange. The radiation from the cavity opening can be felt on the face ten to fifteen feet from the fire. Much of that radiation comes from the front-facing surface of the back log, which glows bright red and looks at you without obstruction through the slot opening. After several hours, the burned out fuel elements are replaced, and can be continually replaced so that the Slot Fire can be operated indefinitely to supply heat steadily. (Chapter 7, Pages 7.1 and 7.2)

Homes have been destroyed by chimney fires. ... One effect of those chimney fires has been to raise the question of the fire-safety of the Slot Fire.

The answer has been so favorable that exceptional immunity to chimney fires has been claimed for the Slot Fire in two letters in the widely respected Fire Journal of the National Fire Association. The argument, consistent with experience to date, is simple. Whether one has much or little creosote on one's flue surfaces, heat or flames are needed to ignite the material to cause a chimney fire. With the Slot Fire, flames are capped from above, and with proper operation should never reach into the flue to ignite any creosote that may be there. In addition, the upper log of the Slot Fire assures that flue gases from the Slot Fire are at much lower temperature than those of a conventional fire. Not a single chimney fire has been reported associated with the use of the Slot Fire. The claim of no chimney fires has appeared in Fire Journal in 1987, and in 1990 and is uncontested. (Chapter 7, Pages 7.2 to 7.3)

The following website contains some brief information about the Texas Fireframe Grate, including information on how to order one:


Click on www.grandpappy.org for Robert's Home Page.

Grandpappy's e-mail address is: RobertWayneAtkins@hotmail.com