It appears you have a zero clearance prefab fireplace. Have you done any research whether or not the manufacturer is still in business?

Also contact your local stove/fireplace shop and see if there's a refractory cement you can use to level everything out.

D K and Sons, thanks the advice.

I bought some fireplace-rated mortar at Home Depot, made by a company Hercules. I troweled down one layer after I cleaned out the fireplace as best I could. The mortar after an hour is soft to the touch - hasn't hardened yet. And after I put it down I used a heat-gun and applied heat uniformly over the area for 10 minutes.

I'm assuming it'll harden soon enough?

Superior is a common name, but from google, they appear to still be in business. I am not a pro, just an old pyro. Even current prefab fireboxes can be pretty cheezy. I doubt that you really needed to do anything. Remember, heat rises. The base is comparatively cool. Level is certainly not an issue. If you were actually using it, you wouldn't have seen the cracks anyway. If it had been me, I would have found a well respected chimney person to check things out. Well respected, because you don't want someone who will tell you that you need a new flue (unless you do, which would likely be a far more serious issue) and new this and that when you didn't.

@chas045, I failed to mention that I my home-inspector who came before I bought the home (who is very sharp but I don't know if he's CERTIFIED to check fireplaces) said I definitely need to do something about the refractory material in its current state. Yes he too said to get someone well-respected. At this point I don't want to dish out the $300 for the initial inspection and more for any repairs. I'm not planning on using the fireplace; however, I may rent the place in 3 or 4 years from now. At which point I may hire a professional. But for now I'm going the repair route suggested earlier in this thread.

When I tried googling Superior, I got a few hits but I did not get the same "Superior" company that's listed on the nameplate and based in California / Baltimore.

I did check the mortar this morning. It still seemed soft but it was a little bit harder compared to yesterday. My buddy who's a contractor said mortar can take up to 40 hours to cure...but he says a lot of things. Lol.

After 4 days, the fireplace mortar I put down is still "soft to the touch", almost clay-like.

Do I need to start a fire in the fireplace to get it to harden?

Any tips?? I want to trowel on another layer of mortar but I don't want to with the state it's in.

Thanks.

Seems like that would have been noted in the product information if it needed heat to harden.

Most refractory products that you can buy at a big box store should state directions of use. I doubt that you need to build a fire and I suspect that you have refractory cement which will not do much for the repair. Take a wet sponge and rub the new material and see if it comes off. Most, but not all refractory cements are water soluble and easily clean up with a sponge. My experience has been that they dry quite hard after a day or two so I always clean up after I finish laying for the day.

Again note that refractory cement is not a good patching material. There isn't really a good patching material on the market, but as I stated earlier Heat Stop makes Insulstick that will do for a while. I have a tub of it on hand and I would be glad to send you a pound or two to at least get you fixed up.

D K and Sons / All:

This stuff still hasn't fully hardened. It's clay-like still. I bought it from Home Depot: http://www.homedepot.com/p/Hercules-1-2-gal-Furnace-Stove-Cement-35515/202078154

Directions of use:

"Surface to be joined or repaired should be clean and free of any old cement or debris. Clean surface and remove paint, grease, or oil before applying furnace cement. Wash cleaned area with water. Do not apply to very hot surfaces; wait until surfaces cool down. Apply thin layer of cement covering all openings. Press tightly into section or part being repaired or sealed. Apply liberal second layer of cement and smooth with trowel. If adding adjoining sections, place gently and avoid any twisting motions. Remove excess cement and smooth edges. Start heat using low temperatures for initial firing. Gradually increase the temperature allowing the cement to cure to a solid and durable mass, free from pores. Full firing may follow. Always keep container tightly covered when not in use."

They talk about a "firing" and using heat. It's unclear exactly what I should do though. Doesn't say anything about how long it would take to dry. I'm confused. Thoughts?

Thanks.

Any thoughts?

Well, simply from those instructions, it appears that you are supposed to build a bunch of fires starting with smaller ones and increasing. I would imagine that a small fire is one that goes for an hour or two with small to moderate wood that never gets roaring hot. Admittedly, that is a little conflicting in that kindling can generate significant heat but you probably wouldn't be generating a big base of hot coals. Later fires could be those that do have that large base of hot coals and a roaring fire above.

However, remember, I'm the one who is not the least convinced that you needed to do anything. I also wouldn't expect that you would need to spend $300 just for a consult from an expert.

Agree with you for not spending $300 for an expert.

It doesn't seem right to build fires on top of the mortar when the mortar isn't even dry enough yet to build a fire. Plain and simple the directions on this package suck. I'm going to complain to the orange big-box store that I bought it from. Also I won't be buying this product again.

I'm not planning on building a fire to dry the mortar. I'm going to attempt to let dry at room temp. Seems like it just takes a long, long time to dry - like greater than a week. At 7 days the mortar has not hardened fully but it is harder than when it was applied. It has a little harder than a clay-like consistency.

Bottom line - it was not my intention to ever use this fireplace. The reason I'm cleaning and patching it up is to make it look "presentable". After I'm done mortaring I'm going to spray-paint the metal panels with black 2000-degree rated spray-paint to make them look nicer.

If a few years down the line I want to start fires in the fireplace, I will first do what D K and Sons said - hire a pro to inspect.

I'm going to try and use a space heater for a few hours and see what that does. No desire to start a fire and see if that works. The substance is still clay-like on the bottom but seemed to have hardened up better in the back of the fireplace.

I would also recommend contacting someone in your city's building department or the fire marshal to make sure it is ok to use before you fire anything up again.