| Many fireplace masons are not familiar with refractory mortar, or they confuse it with fireclay mortar.* Masonry fireboxes are usually laid in ordinary Portland cement mortar, sometimes with a little extra cement in it or perhaps with some fire-clay added to make it "fire-clay mortar".
The problem with this practice 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, (the premixed kind that comes in a bucket and is about consistency of drywall topping compound) uses sodium silicate as a binder which does not deteriorate with heat. In fact, like the firebrick themselves, this refractory mortar takes a ceramic set and gets even stronger when heated.
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, there's another powerful reason to use refractory mortar - it's faster and easier (and therefore cheaper) and looks better.
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
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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.
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