On a non-coated grid, where should the powder particles stick? They will just fall through the grid. You need a supporting film for particles, and usually this is a carbon film because of the good conductivity and easy manufacturing.
Probably, you meant just Formvar-coated grids. The carbon coating stabilises it - sometimes the Formvar films creep under the beam without the coating. The layer of carbon is very thin and it doesn't influence to image quality.
If your particles are more than the grid holes, it is possible to use grids without any film - just put a grid to holder, then put on few particles and cover it with the second grid combining the holes. This way is useful for high-resolution (without artifacts of film) of the grain edges but it's possible to contaminate the microscope and operators usually don't like it.
Good luck!
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