is wood inappropriate for a neon sign powered tesla coil?
The only things in its favor are its inexpensive and high resistivity were as a polymer can become static charged. Wood also has a low combustion temperature. Is there an ideal material?
That's a relief! I had visions of a giant spiral staircase with thousands of turns.
Wood is perfectly adequate as a skeleton for building a Tesla generator but as with all other materials, you need to ensure it is out of range of flash overs. I don't think combustion is a particular concern as most plastics will burn to carbon under the same conditions.
Didn't want to start a different topic but not sure whether stainless or brass screws are ideal for spark gaps. There are lots of stainless steel materials.
For spark gaps it probably makes little difference but for the end point of a continuous arc you should go for the one with highest melting point. Arc temperatures will be very high and if you are looking for a directed discharge path, will be pointed so the heat will be concentrated on a small area.
I'm guessing the next question is "which has the higher melting point?" and the answer is I have no idea. It probably depends largely on the exact alloy proportions in Brass and processing of the steel when manufactured.
If you don't mind spending a couple extra dollars you might consider TIG welding electrodes. They are usually made of tungsten. From what I understand thoriated tungsten holds up better than pure tungsten. Looking on eBay I see a pack of 10 - 1 mm x 150 mm electrodes for around $5 usd. You may find something for even less or if you have a welding shop nearby you may luck out and they give you one. If you decide to use brass screws check to see that it not just brass plated.
Was trying to think where you might salvage something suitable. Many cutting tools use carbide but they usually just carbide tipped. While searching around I came across pine wood derby car weights. The weights are tungsten because of its density. I found a set of four .25" x 1.35" tungsten carbide weights on eBay for 6$ + shipping. If your interested search eBay for " four tungsten carbide rods - pinewood derby car scouts weights.
Im going with bend to shape stainless steel wire and fasten it down with screws and washer for the spark gap. This will permit large parallel surface area and mitigate single point heating as in a nail or screw tip.
Is earthing necessary with a direct earth path for a NST tesla coil?
It doesn't seem it should be since the neutral is directly tied to earth ground and would provide the voltage gradient after attaching one side of the high voltage secondary to neutral but Im not sure.