From what I have read on the experiences of those who have gone before on the home built EDM, they suggest not using distilled water as it quickly becomes conductive, the trend seems to be to use a comercial type industry EDM fluid. But I have heard that some of these comercial types prevent cuts from healing for weeks.
It is also very important to keep the gap flushed of all spark residue.
I believe for a die sinker there is a type of machinable ferrite available.
A usefull book to get hold of is the 'Build an EDM' by Robert Langois published by the Home Shop Machinist ISBN 0-941653-52-8
In the book he ended up using a dielectric made of ethylene glycol, soluble oil, and water to a ratio of 5:5:90. this is cheap enough that you can change it frequently to avoid the conductivity problem.
Al
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.