Artisanal Heating Blankets with Justin Eisenman

Transcript lightly edited for readability — filler and screen-share fumbling trimmed, spellings corrected. Speaker attributions added where clear.

00:06 — Why build your own

Ian Davlin (host): You’re taking nichrome into uncharted territories again. Last time we talked, you’d taken a Les Paul neck off with eight little resistance heaters — and now you’re making your own heat blankets. This has been on my wish list. What brought you to take this on?

Justin Eisenman (guest): Bridge plates, mostly — but I’ve been thinking about it for a long time. My bridge heater is five inches, so it can only heat one wing at a time; I’m always wishing it were an inch longer. My fingerboard-extension heater is too long, so I’m putting cardboard over the soundhole and bending it up, forever fighting to keep heat from going where it’s not supposed to. The store-bought blankets don’t fit everything. All the makers say they’ll do customs, but I’m cheap — by the time you draw it out and go back and forth, the juice isn’t worth the squeeze. So I researched a little and came up with a way to make them in my shop.

03:18 — The materials

Justin: I bought a sheet of high-temp silicone off Amazon — about a sixteenth of an inch thick, and I’d be surprised if it was over twenty bucks; I have enough left for another six or eight blankets. I took a pattern of this ’70s Martin bridge plate with magnets and paper, laid it out, and cut two pieces — the top and bottom of the sandwich.

04:58 — The formula

Justin: Voltage squared divided by watts equals resistance. A foot of 24-gauge nichrome is about 1.6–1.7 ohms, so divide your target resistance by that and you get your length of wire. My storebought bridge heater is 40 watts and gets plenty hot, so I aimed for 40 watts. Designed for the 110 dimmer I plug my other heat stuff into, the formula wanted about 19 feet of wire. So I switched to DC — my bench supply does 12 to 31 volts — and the same math came out to 3 feet. I was kind of rednecky about it, but I did my homework; anybody trying this should do their own. And honestly, with a variable supply and sensible wire spacing, the formula matters less than you’d think — you just turn the knob.

Justin: Laying out three feet took some drawing: both wire ends have to terminate where you choose — at the cord exit — and the path has to total your length. I did a bunch of math I probably didn’t need; quarter-inch spacing and the variable supply would have been fine. One real fix from round one: leave a tab on the silicone to encapsulate the connectors, so nothing live is exposed if you ever run it at 110.

08:40 — The nail loom

Justin: The loom is inch-thick insulation foam board with a thin wood face — thin enough to push nails through, thick enough to hold them. Nails go at every corner of the wire path, the wire wraps around them, and then you cut the heads off. The ends land on two non-insulated butt connectors from the auto-parts store. Lesson learned: put nails on both sides of every tight change of direction — the wire wants to arch where you want it straight, and I had to slice open an edge and push a wanderer back in.

10:26 — The sandwich

Justin: The adhesive is liquid high-temp silicone — the caulking kind, ten or twelve bucks. That’s the peanut butter and jelly. Smear it, wipe it flat with a finger, set the top sheet on, and clamp. Because the nail heads are gone, the clamps squeeze the nails through the sandwich while the silicone grabs the wire; pull the nails and, in my imagination at least, silicone flows into the holes and gives you your electrical insulation. Wax paper on both sides before you clamp, or you’ll spend some quality time regretting it. Cure is about 24 hours, then trim it up with scissors.

12:41 — Running it

Justin: I robbed a power cord off something for the 110 version, then cut that off and went to banana clips into the DC supply. I was teetering around 300°F and backing off to hold about 275 — new thing, so I was cautious — and controlling the heat coming through the top with rags and a bag of ice water. This morning I plugged it in and hit 350 pretty quickly. Encouraging. With less wire and more current it can go hotter.

14:28 — Ian’s LMI blanket history

Ian: In the early days LMI sold a blanket that ran straight into 110 with no controller — a little too hot, but manageable. When they moved to the controller arrangement, the blankets became completely unmanageable plugged straight into the wall — up to 500 degrees and then a Chernobyl meltdown, especially if the thermocouple slipped out and it stopped reading. And when I ran my resistance heaters on the LMI controller, the pulse-width modulation hit them with micro-bursts of full 110 and they went poof — didn’t even get one blast.

21:31 — Thermocouples and heat doctrine

Justin: I monitor with a cheap thermocouple — I fed the probe in through a bridge-pin hole, folded it around, and clamped the pad against it. I started with an aluminum caul behind the blanket thinking it would maintain heat, then scrapped it for wood — the aluminum was acting as a heat sink. What you want is fast, hot heat, and then get out of there.

Ian: Fast hot heat to avoid fun things like letting the center seam go.

Justin: Right. And even so, the top dried out a little — subtle shrinkage between the winter lines, and the belly went very flat. I’ve spent two weeks reintroducing moisture and it’s leveling back out. Heating one big area all at once has upsides and downsides; it’s a process to be refined.

24:12 — The patient

Justin: A ’69 Martin D-35 — big rosewood bridge plate, hollowed out between the pin holes, player’s-grade instrument. I put in a modern-size black locust plate. John Arnold got me onto black locust years ago: under a mirror it looks like old maple, same thickness works, but where maple has that fundamental thing, black locust adds a little extra crunch, bark, and presence — which the bluegrass people I recommend it to, myself included, tend to want out of a dreadnought.

26:36 — Martin’s white glue, and humility

Ian: That era of bridge plate went in with that nasty translucent white glue — super heat-resistant. I fought one in a ’69 D-28 as a young luthier: heat blankets just shrugged off. What finally worked was copper wire from the hardware store with a big nut threaded on the end — torch the nut red hot, nuke a little section of plate until it’s crispy, and pull it out piece by piece. Came out clean. And after all my big talk about the monster bridge plate… the guitar sounded exactly the same. Not better, not worse. Same with the ceramic-bridge Gibsons I used to “rescue” with wood bridges and bone saddles — sounded worse. You just never know.

Justin: The customer’s verdict?

Ian: We didn’t talk about it. The plate was failing anyway, so it needed to happen regardless.

31:38 — Where the loom idea came from

Justin: I saw a forum post where somebody 3D-printed a loom for a heated printer bed, with heat tape underneath. Heat tape was one more thing to research, so I stopped there — but the loom stuck, and headless nails were just a thing I stumbled onto. Worked slick. Total build time is about an hour and a half, less once you stop doing math.

Ian: Could you skip the top sheet and cast the whole thing — 3D-print a mold with channels and pour the silicone?

Justin: Maybe. The caulking silicone isn’t as strong as the sheet — you can tear it — but whether that matters, I don’t know. The top sheet also holds the wires in place, and the closed sandwich was a safety thing for 110. It ends up about twice as thick as a store-bought blanket.

James Rodman (member): There’s dissolvable 3D-print filament, made for supports — print the channel form, lay the wire, glue one side, dissolve the form in a water bath, then glue the other side.

35:25 — What’s next

Justin: A belly-bridge heater — with the belly section I can flip up over a popsicle stick when I don’t need it — a fingerboard-extension blanket, maybe a one-inch spot blanket, maybe a headplate one. Sky’s the limit. A member also pointed out the thin heater tapes on Amazon — small etched-film elements in lots of shapes you could wire in series or parallel — which would probably be even better for some of these.

41:07 — The power supply (and the wall-hanger)

Justin: The bench supply is a Tacklife MDC01 — thirty bucks cheaper than the one from Ian and Trevor’s presentation, and it’s been a workhorse for seven or eight years. Behind me — that’s a 1907 Gibson mandola. It sat on my to-do pile for years; center lamination, headplate, fingerboard, bound the back.

42:48 — War stories

Ian: How long was your pull? My ’69 was a full eight hours and I was pretty unhappy — I budget two to six for that stuff now; more than six and I should go work for UPS.

Justin: About two hours by the time tracker, not counting building the blanket — cutting the sponge, soaking, all of it. Then the black locust work, and the fussing over what the top does after.

Ian: The moisture trick changed everything. Last Martin plate I did — hide glue, a 1940s 000-18 — I laid paper towels in there over several days, not soaking, just insinuating dampness, with the blanket barely warm. Got a fingernail under it, worked guitar picks in up to the pin holes so the metal tools wouldn’t discolor the wood, then pressed down at the center — and the whole plate released out of the tuck in one piece. As early as my Gruhn’s days we were raw-dogging these with pure heat, and if the runout worked against you they could really get away from you.

Justin: And have a drawer of these — spatulas bent into different horrible shapes, one side or the other beveled. This little broken-off one does most of my starting work.