Thermalite Equivalent Fuse

The word "thermalite" has come to be widely used in the pyrotechnics community as a generic term referring to a specific kind of fuse or ignitercord. The commercial version burns very hot and is therefore useful for initiating hard to ignite compositions. One very common usage is as an igniter in amateur composite rocket motors. The fuse comes with external wrappings of nichrome wire which can be used to perform electrical ignition. Sources for thermalite are increasingly hard to come by and purchasing it by mail will usually require permits and licenses. Thus, the need for producing a suitable home-made equivalent has never been greater. The method presented here represents my own adaptation of the method described in a publication entitled "Homemade Imitation Thermalite Ignitercord" by Firefox Enterprises, Inc. This project illustrates the production of only one of the many varieties proposed in the Firefox publication and does not begin to offer the full wealth of information contained therein. Various combinations of binders and chemicals can be used to produce burn rates ranging from 5 seconds per inch to slightly more than one second per inch. Since my most common use of thermalite is for cross-matching time fuse, I chose to illustrate the fastest burning and easiest to ignite variety.

WARNING!! This project uses chlorates and finely powdered magnesium. The dry composition is very sensitive and must be handled with proper care. This procedure should only be attempted by those whose knowledge and experience will minimize the risks of handling these materials.

There are a few pieces of minor tooling that enhance the process of making thermalite, but they are very simple to make. First, a pedestal to hold the batter cup is nice because it frees up the hand that normally would have to hold it. This one can be clamped to the edge of a work bench so that the batter cup is held out over the floor. It is made of pieces of 1 x 4, one of which has a round hole in it for the cup.
Here, I have made a drying rack by using hot glue to fasten 18 clothes pins to another piece of 1 x 4. This is hung from the ceiling of my shop so that fairly long lengths of fuse may be suspended from it if desired.
A binder for the fuse composition is made by mixing the following: (parts by weight)
  • vinyl resin................................. 47 parts
  • nitrocellulose lacquer(10%)....... 25 parts
  • dibutyl phthalate(plasticizer)..... 10 parts
  • acetone.................................... 18 parts
The vinyl resin and plasticizer are available from Firefox. Thirteen bucks will buy enough of both to make more fuse than most people will use in a lifetime. You can mix up a lot of the binder at once and store it in a jar for whenever it's needed.
Now the dry composition is prepared. This is where a good deal of caution and proper handling becomes imperative. The list of chemical ingredients is:
  • potassium perchlorate................. 37 parts
  • potassium chlorate...................... 30 parts
  • charcoal, air float........................ 10 parts
  • magnesium, 200-325 mesh........ 15 parts
  • red iron oxide, ferric..................... 5 parts
  • aluminum, -325 mesh, flake.......... 3 parts
  • sodium bicarbonate(additional)..... 1 part
The magnesium should have been treated with potassium dichromate or coated with linseed oil prior to using it in this composition. This will make it less likely to react with any water present in the acetone. Another alternative is to use molecular sieves to dry your acetone. The potassium perchlorate, potassium chlorate and iron oxide are screened together first. Make sure a screen which has never been used with sulfur is chosen for the screening. In a separate mixing cup, the remainder of the dry ingredients are weighed and stirred together. Finally, the two compounds are mixed together using a gentle method called the diaper method, shown in this picture. The two piles are placed on a large piece of paper, such as newspaper, and mixed by picking up alternating corners to gently roll the powders over each other until thoroughly mixed.
Prior to mixing the binder with the dry powder, the core wires should be prepared. For a typical batch, I cut 18 lengths of 26 gauge copper wire to a length of 19 inches. The wire is then "roughed" by pulling it through my pinched fingertips which are holding a piece of folded sandpaper. This gives the wire surface enough texture to allow the batter to stick to it. Now, 25 grams of dry mix and 17 grams of binder are placed in a 5 ounce paper cup and stirred to make a batter about as thick a pancake batter. The cup is placed in the "batter pedestal" and a small hole is poked in the bottom. The batter should be thick enough that none will drip out of this hole. This "first dip" is accomplished by pushing the wire up through the hole until about an inch of it protrudes from the top surface of the batter. A tissue is used to clean the batter from the top 1/2 inch of the wire. Then the wire is grabbed from the top and slowly pulled the remainder of the way through the batter. After the first dip, the batter diameter will be quite small at about 1/16 of an inch. The wire is then clipped into the drying rack and the procedure is repeated for the remainder of the wires. They will be ready for the second dip in a few hours.
The second dip is identical to the first, except that the hole in the batter cup may need to be widened slightly to allow the thickened first dip wires to go through. The batter can be slightly thicker for the second dip so that the coating will be thicker also. Pulling speed will also affect the finished thickness somewhat. Very slow pull speed allows some of the batter to drain off, yielding a thinner coating. Both of these factors can be varied to adjust the final diameter between the range of 1/8 inch to 3/16 inches. It may be necessary to add a few drops of acetone and stir the batter occasionally because this solvent evaporates so quickly. Again, the dipped wires are clipped to the drying rack until they are completely dry. This will take about 6 to 8 hours.
When completely dry, I cut the finished igniter cord into 6 inch lengths so they can be easily stored. The finished product is firm, but flexible. It is very durable because the composition will not flake off easily when the cord is bent. This stuff is quite water proof and burns with a pleasing and loud hiss. I have found a plethora of uses for this marvelous improvisation for thermalite. Among them are:
  • It can be used as a substitute for visco in most cases, although the slower speed variety is better for this purpose. At 3 cents a foot for materials cost, it's less expensive, too.
  • It is a very good replacement for black match when used for cross-matching.
  • I use a small length of it in the nozzle of fountains when I want to ignite many at once with sticky match. The same concept is used to ignite girandola drivers and most other forms of black powder rockets.
  • Whenever I use sticky match to replace quick match for mine and shell leaders, I put a piece of the thermalite at the end of the match to assure ignition of the lift powder.
  • I haven't tried this yet, but I think there is a good possibility that short lengths of this cord could be used to make go-getter inserts in a shell. A few wraps of masking tape with a little bit of one end left exposed, would create very energetic little rockets. Even unwrapped lengths will fly all over when lit if they are not attached to something.
I'll close this project page with a few comments about the Firefox booklet. If you are seriously contemplating this project, I highly recommend that you purchase the booklet. It contains two different dry formulations and two binder solutions. It will tell you how to make this thermalite equivalent in a variety of ways to achieve a wide range of burning speeds. I have not included all this information in my project description because Firefox deserves to benefit from their development efforts and publication of this material.

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