Slate Headstones: Why These 1700s Markers Survive and How I Clean Mine

headstone cleaning in Massachusetts | headstone cleaning in Boston Massachusetts

I remember the first time I laid eyes on a slate headstone in our old cemetary. It was a gray, drizzly afternoon back in 2009, and I was just starting out as a volunteer. I tripped over a fallen branch and landed right in front of a tiny slate marker from 1742. The name was almost gone, but the carving of a weeping willow was still sharp as a tack. I sat there in the mud, just staring. That stone had been through two hundred and sixty-odd winters, and it still held it's story. Took me a while to get up. My knees aren't what they used to be.

Slate is a funny stone. It's not flashy like marble or fancy like granite. But it's tough. I've cleaned dozens of them now, and I keep coming back to slate because it rewards patience. If you're looking at a gray or black marker from the 1700s or early 1800s, odds are it's slate. And if you want to clean it without wrecking it, you need to know a few things. I've made my share of mistakes over the years. Let me save you the trouble.

Why Slate Survives When Other Stones Crumble

Slate is metamorphic rock. It started as shale, then got cooked and squeezed deep underground. That proccess gives it layers, like a stack of paper. That's why it splits into flat sheets — perfect for carving letters and pictures. But here's the thing: those layers also make it vulnerable to water. If water gets between the layers and freezes, it can pop the surface off. That's called delamination, and it's the main reason old slate markers get flaky.

Still, slate resists acid rain better than marble or limestone. Marble is calcium carbonate — it dissolves in acid. Slate is mostly quartz and mica. Not much calcium. So the old carvings stay crisp for centuries if the stone is good quality. I've seen slate from the 1690s that looks like it was carved last week. Not all slate is that tough, but the best stuff is nearly indestructible. The worst stuff? It's crumbly and soft. You never know until you touch it.

What I Use to Clean Slate Headstones Safely

I've tried alot of things over the years. Some worked. Some didn't. Here's my go-to list:

First, water. Lots of water. I carry a five-gallon jug and a soft spray bottle. No pressure washer — that's a disaster waiting to happen. Slate can flake off in sheets if you hit it with too much force.

Second, D/2 Biological Solution. That's the only commercial cleaner I trust on old stone. It's pH-neutral, non-ionic, and it kills biological growth like lichen and moss without eating the stone. I buy it by the gallon. Expensive, but worth every penny.

Third, soft bristle brushes. I use a natural-bristle scrub brush (not nylon — that can scratch). And for tight spots, a soft toothbrush. Nothing metal. No wire brushes. Ever.

Fourth, wooden scrapers. I whittle my own from popsicle sticks. Great for lifting off thick moss without gouging the slate.

That's it. No bleach, no vinegar, no muriatic acid. Those are for cleaning concrete, not history. Slate doesn't need harsh chemicals. It just needs patience.

My Step-by-Step Method for Cleaning Slate

Alright, here's how I do it. I'm not a professional. I'm a retired schoolteacher with a bad back. But this works.

Step one: Wet the stone thoroughly. I start at the top and work down. Let the water soak in for a few minutes. This softens the dirt and biological gunk.

Step two: Apply D/2. I spray it on evenly. Don't drench it — just a good coat. Let it sit for five to ten minutes. If it's hot and sunny, I work in the shade or wait for a cloudy day. Direct sun dries the cleaner too fast.

Step three: Gently scrub with a soft brush. Circular motions, light pressure. I focus on the carved areas. The letters and designs hold dirt. Be careful around flaking spots. If a piece looks loose, I don't touch it.

Step four: Rinse with clean water. I use a spray bottle or a gentle hose nozzle. No high pressure. I rinse until the runoff is clear.

Step five: Let it dry. Slate dries fast, but I come back the next day to check for missed spots. Sometimes I need a second application. That's fine.

Big mistake I made once: I scrubbed too hard on a flaky slate from 1765. Took off a chunk of the date. Now it just says "17—". I still feel bad about that. So go slow. You can always do more later.

What to Do About Lichen and Moss on Slate

Lichen is the enemy. Those crusty orange or gray patches dig into the surface. They hold moisture against the stone. Over decades, they can pit the slate. Moss is less aggressive, but it still traps water.

For lichen, I use the D/2 and a wooden scraper. Spray it, wait, then gently scrape. Don't pry. If the lichen is thick, I might do three rounds over a week. It comes off eventually.

For moss, I spray with D/2 and let it sit. Then I use a soft brush to sweep it away. Moss roots are shallow. Usually comes off easy.

Never use bleach. Bleach kills the lichen above ground but leaves the roots. It also whitens the stone unevenly. I've seen slate that looks like a calico cat after someone bleached it. Not pretty.

When to Leave a Slate Headstone Alone

This is the hardest lesson for a volunteer. Sometimes cleaning does more harm than good. If the slate is delaminating — if it has loose flakes or bubbles — I walk away. I might wipe off surface dirt with a damp cloth, but no scrubbing. No chemicals.

I also avoid cleaning if the stone is wet from rain or dew. Slate is more fragile when saturated. Wait for a dry day.

And if the carving is so worn that you can barely read it, don't try to "bring it back" with aggressive cleaning. You'll just lose the last traces of the inscription. I've seen it happen. Sad.

Some stones are meant to fade. That's not failure. That's time doing its work. My job is to slow it down, not stop it.

Frequently Asked Questions

Q: Can I use vinegar to clean a slate headstone?

A: No. Vinegar is acidic. Even diluted, it can etch slate over time. Stick with pH-neutral cleaners like D/2 Biological Solution. It's safer and actually works better on biological growth.

Q: How often should I clean a slate marker?

A: Once a year is plenty, unless you see heavy lichen or moss. Overcleaning wears the surface. I do mine in early spring, before the growth season starts. That way the stone stays clean through summer.

Q: What if the slate has flaking or peeling layers?

A: Stop. Don't scrub or spray. Flaking means water has gotten between the layers. Cleaning can make it worse. Consult a professional conservator if the stone is historically importent. Otherwise, just leave it alone.

Q: Is it safe to use a pressure washer on slate?

A: Absolutely not. Pressure washers can blow off the surface of slate, especially if it's already weathered. Even on low pressure, the force is too much. Use a spray bottle or a gentle hose with a soft spray nozzle.

Q: Can I use dish soap on slate?

A: I wouldn't. Most dish soaps have degreasers and fragrances that leave a residue. That residue attracts dirt and biological growth. Plain water or D/2 is best. If you must use soap, choose a mild, fragrance-free, pH-neutral one. But honestly, water and a soft brush do the job.