If you are working on the bricks or hearth of your fireplace, you will have the need to mix fireplace mortar. Fireplace mortar is mixed a bit differently than regular mortar.
Step 1 - Mix the Mortar
One of the most important parts of mixing fireplace mortar is to purchase the correct type of mortar. Since the mortar will be exposed to heat you can't use standard mortar. The heat will cause it to crack and crumble. You can purchase refractory mortar at most building supply stores and fireplace shops. It can also be purchased on line.
The proper mix is 6 parts mortar, 1 part lime, and 1 part sand. Put this in a large container and stir it to combine all the dry ingredients. Then slowly add water to the consistency you need. If you are working on a flat surface, such as a fireplace hearth, it can be a thin, slurry mix that you can dip the backside of the brick into then push into place. Bricks to be placed on a wall need to have a thicker mortar.
Step 2 - Finish and Use
You will find that this type of mortar dries to a creamy white color. It will darken with use, or you can stain it if it's used on a hearth. Allow the mortar to dry at least one week before using the fireplace.