How to make cheap big cores?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Tony Bryant by way of Terry Fritz <twftesla-at-uswest-dot-net>" <brd-at-paradise-dot-net.nz>

Hi, I'm looking for a way to get a big HF transformer cores.

Allstar magnetics have the perfect TDK EC120 cores in stock, but 
at the price of US$157ea for 10+, puts it way way outside my budget.

Basically I'm looking for 3KW throughput. Its for an inverter welder/
plasma cutter (yes I know this is the Tesla list, but there must be some 
overlap here r.e. transformer cores).

As an experiment I obtained some magnetite (black beach sand),
cast it into a core. Unfortunetly no measurable increase in permittivity 
(or is that permability?) over an air core.

Next experiment will be to make an iron powder core. 
I intend to get some powder from a metal spraying supply place,
dunk it in H3PO4 to get a black oxide layer, and then cast into
a big toroid with a small amount of epoxy.

Is this feasable?