Excerpted from “The Tesla Phile” newsletter of May 1985 written by
the late Peter Lefferts. Provided by Gerald O’Docharty Feb. 1994
BOLT BARRIER, compound for Tesla coils
To have an easily castable compound for insulation that is tough yet
slightly resilient so that stress cracks will not occur. Available
kitchen materials and low melting point are very important. My
primary use will be to cast insulation between the primary and
secondary using cardboard molds. The object is to wind the primary
closer to the secondary, depending on the insulation to prevent
flashovers between the two windings.
Paraffin, shredded soft polyethylene (old, translucent, coffee can
lids etc.) (maybe clean clear polyethylene sheeting commonly
available. G.O’), salt, & two large pots… one of which will sit
easily inside the other. These pots should be stainless steel or
good uncracked glass-porcelain covered.
First step is to make up a high temperature double boiler where the
inner pot sits in the water. Some sort of support is needed so the
inner pot doesn’t sit on the bottom. Put enough paraffin blocks in
the inner pot to supply the expected need. The inner pot will have
to be weighted down because the outer pot should have enough water
added to come about an inch above the melted wax level.
Boil the water while adding salt to it until no more salt will
dissolve. this raises the boiling point for a better product, yet
greatly decreases the chance of flames.
When the water is at a medium boil with most salt dissolved and the
wax melted, start adding the slivers of flexible polyethylene
(avoiding other plastics that are stiffer or make a white line when
Stir every few minutes to prevent the poly from sticking to the
bottom and browning the wax. When about 1/2 the poly appears melted,
add some more. Add water slowly to the outside pot to keep the level
an inch higher than the plastic level of the inner pot.
As soon as a hint of cloudiness appears in the mixture, or the
consistency of thin oil appears, stop adding poly, and heat and stir
a little longer.
A mold of good dense cardboard, glued watertight sides and bottom
with ample epoxy should be prepared well ahead of time, also seal
any wire joints likewise. Do not use other plastics such as Acrylics
as molds, practically all epoxies and hard magnet wire insulations
will stand up to the heat and solvent characteristics of this
Pre-heat the mold cavity using an oven for small objects, or hair
drier for larger ones. If oven, pre-heat it 250 deg.F. and wait
until well stabilized before putting object in. If hair drier, heat
until a metal part will sizzle slightly when touched by a wet
Be sure that if a Tesla primary is being molded that several taps
for tuning are brought out through the mold. Also if you want to
take the cardboard off later, smear the inside with silicone grease
(easier to paint the cardboard though).
Pour rapidly without splashing; this stuff burns skin worse than
boiling water. Shake gently to bring up bubbles. If a small leak
appears, patch with magic tape and blow with hair drier on cold
setting until the plastic in the leak has well solidified.
Bolt barrier is at least an 8 times better insulator than air. It
can be carved quite easily and very smoothly, and in fact may make
an excellent amateur’s sculpture media.
If done slowly, the material can be drilled and sawed with ease. It
makes good light duty custom bearings for models etc. Almost any
household solvent may be used for short term cleaning without
effect…turpentine has some polishing action.
If you have comments or other information relating to such topics
as this paper covers, please upload to KeelyNet or send to the
Vangard Sciences address as listed on the first page.
Thank you for your consideration, interest and support.
Jerry W. Decker………Ron Barker………..Chuck Henderson