Pour a half gallon of linseed oil into a pot and bring it to a boil. Allow the linseed oil to cool, then pour it into a bucket and mix it with half a gallon of mineral spirits. Mix the mineral spirits and boiled linseed oil thoroughly.
Dip a cotton rag in the linseed oil and mineral spirits mixture. Wipe the liquid on the bricks in your fireplace. Allow it to soak in for about five minutes.
Wipe away any excess mixture from the bricks with a soft cotton cloth.
Apply the boiled linseed oil and mineral spirits mixture to the grout between the fireplace bricks if they appear much lighter than the bricks themselves. Keep applying the mixture until the entire fireplace has a uniform color.