What Mortar and Setting Compound Conservators Actually Use
headstone cleaning in Pennsylvania | headstone cleaning in California
You know, I still remember the first time I tried to fix a fallen headstone. It was a little Civil War marker for a boy named Elijah, died 1863, aged just 17. The stone had snapped clean off it's base—probably a careless mower or a frost heave. I was so eager to put it right. I grabbed a bag of regular Portland cement from the hardware store, mixed it up, and slapped it on. Looked good for about a week. Then came a hard rain, and the whole thing toppled over again. Took me three tries and alot of embarrassed phone calls to a real conservator before I learned my lesson.
I’ve been volunteering at Oak Hill Cemetary for fifteen years now, and I’ve seen every kind of botched repair you can imagine. Cracked bases, stained marble, stones that look like they’ve been glued together with toothpaste. The problem is almost always the same: people use the wrong mortar or setting compound. They grab whatever’s cheap and easy. Big mistake.
So let me tell you what the pros—the conservators who do this for a living—actually use. It’s not glamorous. It’s not expensive in a fancy way. But it’s specific. And it works. If you’re fixing a family stone or volunteering at your own cemetery, you need to know this stuff. I’m going to walk you through the mortar and setting compound conservators use, straight from my own messy, trial-and-error experience.
Why Regular Cement Ruins Old Stones
First off, let’s get this straight. Ordinary Portland cement is too hard. Way too hard. It’s also full of soluble salts that leach out over time and stain the stone. Worse, it doesn’t breathe. Old marble, limestone, and sandstone are porous—they need to let moisture escape. Cement seals them up like a plastic bag. Then water gets trapped inside, freezes, and pops the stone apart from the inside. I’ve seen it happen to a beautifull 1870s granite obelisk. Some well-meaning person filled the crack with cement. Two winters later, the whole top half was scattered on the ground.
So what do conservators use instead? They use a lime-based mortar. Specifically, a natural hydraulic lime mortar (NHL). It’s softer, more flexible, and allows moisture to move through it. It’s what the original masons used a hundred years ago. You can buy it pre-mixed from specialty suppliers—brands like St. Astier or LimeWorks.us. It comes in different strengths: NHL 2, NHL 3.5, and NHL 5. For most cemetery work, NHL 2 or 3.5 is perfect. NHL 5 is for heavy structural stuff, but you rarely need it on a headstone.
The Right Setting Compound for Reattaching Broken Pieces
Now, sometimes a stone isn’t missing a base—it’s just cracked or broken into a few big pieces. For those repairs, conservators don’t use mortar at all. They use a setting compound. And no, not epoxy from the hardware store. The good stuff is a two-part polyester or acrylic resin. The brand I see most often is Akemi, specifically their stone repair kit. It’s got a filler powder that matches the stone color. You mix it, apply it fast (it sets in about 10 minutes), and then you can sand it down once it cures.
But here’s the trick: you have to pin the pieces together, too. Conservators drill tiny holes into both faces of the break, then insert stainless steel or fiberglass pins before applying the resin. This gives the repair real strength. I learned this the hard way when I tried to glue a broken finial back on with just the resin alone. Held for a year. Then a squirrel jumped on it and off it came. Never again. Now I always pin it first.
What You Absolutely Should NOT Use
I need to be blunt here. There are products you should never, ever use on old stones. Avoid harsh household chemicals and abrasive tools—they’re too aggressive. Steer clear of strong acids or alkaline cleaners, even if a well-meaning friend recommends them. I’ve seen marble turn to crumbly sugar because someone scrubbed it with acid. That’s permanent damage.
Also, skip anything that says “waterproof” or “sealer.” Sealing an old stone traps moisture inside. Bad idea. And never use a wire brush on marble, limestone, or sandstone. The metal scratches the surface and grinds away the patina. If you need to clean a stone, use a pH-neutral, non-ionic cleaner like D/2 Biological Solution. It kills the lichen and moss without harming the stone. Spray it on, let it sit for 15 minutes, then rinse with a gentle spray of water. That’s it.
How to Mix and Apply Lime Mortar the Right Way
Alright, you’ve got your NHL 2 mortar. Now what? First, wear gloves and a dust mask. Lime is caustic when wet. Mix it dry with clean, sharp sand—about one part lime to three parts sand. Then add water slowly. You want a consistency like peanut butter. Not runny. Not crumbly. Let it slake for 10 minutes before using.
When you apply it to a broken base, first wet the stone surfaces with a spray bottle. A dry stone will suck the water out of the mortar and weaken the bond. Then butter both surfaces with the mortar, press them together, and support the stone with wooden wedges or sandbags. Let it cure for at least 7 days in cool, damp wheather. If it’s hot and dry, cover it with wet burlap and plastic. I once forgot to cover a repair during a heatwave. The mortar dried in one day, then cracked like a dried riverbed. Had to chip it all off and start over. Took me two full afternoons. Never again.
A Few Words on Repairing Marble vs. Granite
Different stones need different treatment. Marble is soft and reacts badly to any acid. Granite is harder and can tolerate a bit more scrubbing, but it’s still porous. For marble, always use the gentlest method: a soft brush and D/2 cleaner. For granite, you can use a stiff nylon brush, but still no wire or metal.
Mortar, marble repairs work best with a lime mortar that has a fine aggregate—like crushed marble dust instead of sand. You can buy marble dust online or from a stone supplier. It makes the mortar blend in with the stone better. Granite is less fussy, but still needs the lime mortar, not cement. And if you’re repairing a polished granite surface, you’ll need to match the color carefully. I keep a little notebook with color samples from each repair. Sounds obsessive, but it saves headaches later.
Frequently Asked Questions
Q: Can I use regular tile grout instead of lime mortar for a headstone repair?
A: No, please don’t. Tile grout is too hard and contains Portland cement. It’ll crack the stone or trap moisture. Stick with natural hydraulic lime mortar. It’s the only safe choice for historic stones.
Q: Where can I buy the mortar and setting compound conservators use?
A: I get my NHL lime mortar from LimeWorks.us or at a local masonry supply shop that carries St. Astier. The Akemi resin kits I order online from a stone restoration supplier. You can also check with a local monument dealer—they often sell small bags of lime mortar for repairs.
Q: How long does a lime mortar repair last?
A: If mixed and applied correctly, a lime mortar repair should last decades. I have repairs I did ten years ago that still look solid. But it depends on weather exposure and the stone’s condition. Check it every spring and re-point any cracks that appear.
Q: Is it okay to use epoxy glue for broken headstones?
A: Only if it’s a stone-specific epoxy like Akemi’s. Regular household epoxy is too brittle and can yellow in sunlight. Stone epoxies are formulated to be UV-stable and flexible. And always pin the pieces—epoxy alone isn’t strong enough for a heavy stone.
Q: Can I just re-set a leaning headstone by pouring concrete around the base?
A: Oh, please don’t. Concrete is too hard and won’t bond with the stone. It also holds water against the base, causing rot. The proper way is to dig out the old base, set the stone in a bed of crushed stone for drainage, and then use a lime mortar to point the joint. It’s more work, but it’s the right way.