This is not surprising since the major building codes have been unclear, inconsistent - or silent - on the subject. The ICC codes, which are just now being adopted in many states, require refractory mortar for the construction of fireboxes, smoke chambers and flue linings but a short while ago only the NFPA 211 code called for "refractory mortar (ASTM C199, medium duty)". The BOCA code required "medium- duty fire-clay mortar, the UBC just required that the "joints in firebrick shall not exceed 1/4 inch." and the CABO One and Two Family Dwelling Code did not specify the type of mortar or size of joint to be used.

The problem with using ordinary mortar is that Portland cement can't take the heat. Oddly, Portland cement retains its strength up to fairly high temperatures but deteriorates as the temperature cools down through about 600 degrees F. Eventually all that's left of the mortar is the sand and fire-clay with no cement binder. The mortar has no strength and easily falls out of the joints - especially if they're wide joints.

Refractory mortar, on the other hand, is made with high temperature cements and carefully selected aggregates that don't expand and tear the mortar apart when heated.

The International Residential Code (IRC) and the International Building Code (IBC) which have recently been adopted in nearly all states require that fireboxes, smoke chambers and flue liners be laid with refractory mortar conforming to ASTM C 199.

Admittedly, fireplaces are not often used to heat with these days. Fireboxes laid in ordinary Portland cement mortar may last for years if they're only fired up at Thanksgiving and Christmas. But, never mind that refractory mortar performs better, looks better and is easier to use, there's another powerful reason to use refractory mortar - it's required by code.

There are basically two kinds of refractory mortar conforming to ASTM C 199 and permitted by code. Hydraulic-setting mortar sets up, or cures, like Portland based mortars rather than drys. Premixed or air-drying mortar does dry. It comes in pail about the consistency of drywall compound ready to use - except that you might have to thin it with a little water.

Hydraulic-setting refractory mortar

Hydraulic-setting refractory mortar is the best all-around refractory mortar. You can use it to lay the firebrick, set or parge the throat and smoke chamber and set clay flue liners. It has the workability of ordinary Portland-based mortar and can be made almost any color using ordinary mortar color.

Hydraulic-setting refractory mortar is the only mortar that should be used to set clay flue liners, Once it has cured, hydraulic-setting mortar becomes water insoluble and acid resistant. Premixed mortar will dissolve in water even after dried and could wash out if the flue gets wet. Because hydraulic refractory mortar is acid resistant and water insoluble it's the only mortar for any clay flue that will vent a gas or oil appliance.

Hydraulic mortar works better in wet climates where the premixed mortar takes a long time to dry and sometimes leads to efflorescence. Bottom line, hydraulic-setting refractory mortar is an economical and inexpensive all-purpose refractory mortar.

Premixed refractory mortar

Premixed refractory mortar is made with sodium silicate as a binder which does not deteriorate with heat. Because the pre-mixed refractory mortar is not a hydraulically setting mortar (it dries out rather than cures by chemical reaction like ordinary mortar does) the mason can make very thin joints and doesn't have to pre-soak the firebrick. With a 1/16th to 1/8th inch joint and dry firebrick the firebox can be laid rapidly. Ten seconds after setting a brick it takes some effort to dislodge it and this fast drying out of the mortar does not compromise its strength like it would a hydraulically setting mortar.

"I build my Rumford firebacks straight", says Buckley, "but pre-mixed refractory mortar makes building even a curved or slanted-back firebox a breeze because you don't have to form it or wait for the mortar to set up."

Premixed refractory mortar is messy - but remember, it's water soluble, so it cleans up easily with water. Don't waste time trying to be neat. Just plan on washing the firebox down when you're finished.

"I butter a thin layer of the refractory mortar on the firebrick I'm going to lay," says Buckley.

He uses a small square margin trowel because it fits in the bucket of mortar better than a pointed brick trowel. The firebrick is then laid with a minimal 1/16th inch joint and the excess mortar scraped off with the margin trowel.

After the firebox has been completed - remarkably in only about half an hour - Buckley adds refractory mortar to any voids, smearing mortar heedlessly on the faces of the firebrick and making a perfect mess. (He chooses a time when the owner is not watching.) Then he washes the firebox with clean water and a sponge and it looks beautiful.

"In cold or damp conditions, however, the hydraulic-setting refractory mortar that comes dry in pails or bags and is mixed with water just like regular mortar, is a better all around refractory mortar", says Buckley.

The premixed refractory mortar sometimes causes efflorescence if it stays wet too long. The premixed is still my refractory mortar of choice in hot dry climates where the hydraulic mortar would dry out too fast if you don't keep the bricks wet, but "I live in Washington" and the hydraulic-setting refractory mortar is more versatile. "It's not water soluble and it can be used to set the throat, smoke chamber and flue liners as well as to lay the firebox."

Buckley building a firebox. Click for a larger image.