My wife wants to put the glass tiles over the brick around our fireplace (vertical and horizontal surfaces) can I thin set the tile straight to the brick or do I need to use hardy backer first?

First off are the bricks painted ??

If not,dont worry about hardi backer.Just "skim coat" the bricks.Mix a modified thin set(has latex built in) with a 50-50 mix of water and latex ad-mix.It will make the thin set stronger and will adhere better.

Use the flat side of your trowel and apply thin set over bricks and joints to give you a flat surface.

Same is true even if painted ,just rough up surface as much as possible.

No they're not painted. Thanks for the info.

Kingfisher,are you covering the whole fireplace in tiles, or just putting them in random places?

The whole thing, ground up. The brick stops at the mantle so I won't go any higher than that.

Kingfisher I take Wall Mud and use float strips to mud the front of brick fireplaces to get them plumb and level.

This would be to hard for someone who has no experience hanging mud.I woulg get some multi-purpose gray thinset and skim coat the brick until you have a pretty flat surface and let dry overnight.

I use white thinset or mastic to apply glass tiles,since you're installing it over brick I would use white thinset.The reason for the white thinset is that glass tiles are transparent and if you use gray thinset on some of them when it dries it will change the color of the tile.

Also,do not apply glass tiles directly with a notched trowel when the thinset dries you will see the grooves through the glass

I spread the area out with a 1/4 inch notched trowel and then take your margin trowel and back butter the glass tile with a smooth skim coat of white thinset and then apply to your troweled area.

Thanks for the info and advice Jared.