What mortar mix for fireplace?

[BML]

I was a bricklayer many years ago but left the trade and it's a very long time since I built a fireplace so I'm looking for some advice.
The fireplace opening has been stripped out leaving a large box like opening. I want to run a screen of bricks round both sides and the back and then place a fire basket with a cast iron back plate in the finished opening and burn logs.
I think that as long as I use a decent brick and lay them with lime mortar it should be alright but I can not remember what mix it should be.

 

[stuart45]

The mix should be 3/1 sand/lime. This ensures that every bit of sand is covered by the lime binder.

 

[^woody^]

6:1 is strong enough, you could even go to a 9:1:1 if you use cement too.

3:1 may be to strong and more likely crack with the heat

Make sure to iron the joints smooth as you can

 

[stuart45]

I have used a 3/1 mix on quite a few fires over the years and never had any problems as lime mortar is pretty flexible. However a weaker mix may be better, but I havn't used one so I couldn't say.

 

[stuart45]

A 1/1/9 mix is 4 1/2 to 1. Cement will help the mix go off but its flexibility will be less than 3/1 sand lime which is the traditional mix for chimneys.