Oven cleaner (Easy-Off) works very well. Test a small area first before doing the whole thing.

i just wanted to add that oven cleaner really does work.....very little effort with a brass brush !! thanks, Xanndra.

USE A STIFF BRUSH CORN OR SOMTHING LIKE THAT BUT INSTEAD OF MURIATIC ACID THERES A PRODUCT CALLE SURECLEAN WHICH IS LESS ABRASIVE TO THE SKIN MIX WITH THEY CLAIM 10 PARTS WATER BUT YOU CAN MIX IT STRONGER USE RUBBER GLOVES AND EVEN IF YOU GET SOME ON YOUR SKIN IT WILL NOT BURN AS MUCH AS THE OTHER ACID .. BUT IF OVEN OFF WORKS THAT MAY BE A BETTER WAY.. ALSO WITH ACID YOU HAVE TO WET IT DOWN AND RINSE IT WELL OTHERWISE IT CAN BURN THE JOINTS OF MORTAR..

acid is best used only by pros and mainly outdoors. i have used it myself for a few projects, and it does tend to burn the skin and eyes.

try to other solutions first, if they do not work then wait until the weather is nice enough to open all the doors and windows before you use acid inside. i always used a sponge to dip in the acid, then wipe the wall, soaking it. let it sit a minute or so, then scrub, then rinse. you MUST rinse it well, so be prepared for water everywhere. put down plenty of towels and change them out often.

The best way to do it is with Paint N Peel, non toxic-very easy-simply paint it on, let sit for atleast 8 hours, then peel it off. Check it out at www.ahealthyhearth.com

Here is a link that might be useful: A Healthy Hearth