Building a brick arch

[Keefa]

Guys,

I'm in the middle of reinstating a fireplace in my house.
I have laid the concrete base hearth and i intend to tile this with quarry tiles. I am going to use a simple cast iron log basket.
The original brick opening is too large and so I have narrowed the opening by building mini brick piers either side, and i intend to lower the arch at the top. There is already an arch in place holding the weight, but I wish to add another arch below this.
Can anyone give me some tips on how to do this? I tried today by building a temporary timber support but i found that as I got nearer to the centre of the arch I kept moving the bricks already laid. Is there a trick to this? Should I use a special (stronger) mortar mix?
Any help greatly appreciated as i promised the missus she would have a working fireplace for Christmas!

Keefa

 

[Spud1]

I cannot understand you completely, but there are many ways to strike an arch, axed, gauged, rough and so forth. You need to determine what type Segmental, semi, gothic and so forth. You will most definately need to build a centering and will have to decide you rise and springing points so as to form a striking point on the centre rod. It sounds more complicated than it is for such a small arch (don't know the span) and once you know a few fundemental rules, you are away.