Here are the results of my experiments with a recipe
for potting compound which was posted earlier, together with
the original post. I think my method will work as well and
is quite a bit easier to use, so thought it was worth posting.
Also sending a copy to the hvlist, even though I think most
fellows are on both tesla and hv.
Here is a modification to the "Bolt Barrier compound" recipe attached
below from an earlier posting; I think it may be easier to use. I did an
experiment here to see what the stuff was like and how easy it was to mix
up. Here is how I went about it:
1. Put 1/4 pound of paraffin (brand name Parawax, but that shouldn't
matter) in a can and not wanting to get my wife mad by using her
double boiler (besides, didn't want to mess with the watery mess(,
set it on an electric hot plate.
2. When the paraffin had melted fed cut up coffee can lid material into it.
Wassorta surprised to see that it did indeed dissolve. Kept stirring (with
wooden popsicle stick) and adding more polyethylene coffee can lids
until I was all out. It was obvious that more could be dissolved, so -
remembering that hot melt glue is supposed (and almost certainly is)
LDPE, fired up the glue gun and loaded it with Stanley "Dualmelt" glue
sticks, which are supposed to be "long hardening" and thus presumably
low melting point.
3. I dribbled hot melt glue into the paraffin/polyethylene mixture and
kept stirring with the wooden stick. At some point I noticed that the
stick was turning brown, indicating I had let the stuff get way too hot.
Quote from below:
"Available kitchen materials and low melting point are very important."
I was obviously ignoring the low melting point. I poured the stuff
I had already mixed into a Saeco lead mold I have and started over,
using a thermometer as the stirring device. First melted the paraffin
(1/4 pound - I'm too cheap to waste more) and heated it to about 210!F,
then turned off the hot plate and started dribbling hot melt glue into
the molten paraffin while watching the temperature. The hot glue was
warm enough to keep the mixture liquid, and as I kept adding stuff I
was able to dissolve four "4 in x 0.45 in" glue sticks before the stuff
wanted to freeze at about 210!, the limit of what I thought was reasonable.
At this point the mixture consisted of 1/4 pound paraffin and 42 grams
of hot melt glue. (How's that for mixed units?)
I poured it into paper cup molds and let it harden. The mixture has
a pleasant grey color, and unlike paraffin, does not crumble when you
scrape it with a finger nail. I tried putting some of it hot onto a
GM "high-energy" ignition coil I was considering potting, and found
that it appeared to wet the surface and stick very well indeed, at
least as well as paraffin and apparently better. My conclusion is
that this ratio is about right, and that the stuff is suitable for handling
without outside protection, although that would be better.
Bottom line is that this appears to be good stuff, this method of
mixing is easier than the double boiler and probably about as reliable,
and that by using the hot melt glue you save the trouble of collecting
the polyethylene scrap and cutting it up. (Nothing wrong with that,
of course, if you have it on hand.) Stuff is not cheap, probably at
least a couple of dollars a pound, but then our hobby isn't cheap
Here is the original recipe, which I am sure will work just fine too:
"Well, I dug out the original post that has the poly/wax recipe that I had.
I'm posting it in it's entirety below:
Message 5107 DATE/TIME: 02/27/94 17:03
>From : GERALD O'DOCHARTY
To : BERT POOL
Subject: Bolt Barrier compound
Folder : A, "Public Mail"
Hi Bert! Here's the recipe just like Pete wrote it. I have not tried it
yet but plan to do so soon. I am also uploading this as a file.
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
PURPOSE: 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.
MATERIALS: Paraffin, shredded soft polyethylene (old,
translucent, coffee can lids etc.)
(maybe clean clear polyethylene sheeting commonly
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.
PREPARATION: 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.
COMPOUNDING: 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 creased). 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.
CASTING: 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 compound.
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 finger. 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