Any Geologists out there?

Preferred title would have been: “Danger Will Robinson! Danger!” But this one will lean toward the slightly more productive.

Random shower thoughts this morning: How much Chlorine is in random rocks you pick up to etch?

Lots of people have been talking about putting rocks in the Forge. And one type of rock is rock salt. But a salt is just an acid/base pair where the hydrogen in the acid is mostly supplanted by metal from the base.

A quick search online says the earth’s crust is 0.02% or so Chlorine. But… is there any type of rock which is high in chlorine, and should be avoided in the laser?


I am not a Geologist, but rather a rock hound with an interest in chemistry.
There are some chlorides and more complex chlorine containing forms that occur naturally, but since the vast majority of Chlorides are very soluble in water they don’t usually stick around long outside of desert areas. Lead chloride is a fairly common exception (in volcanic materials) and there are others. A bigger source is a fairly common mineral called Apatite, which occurs in small amounts in a lot of places (like in your bones!) but only some forms of Apatite have chlorine in them.

In general I would guess (just a guess) that the amount of Chlorine in any random rock you find will be below the danger threshold, especially since you are not going to be able to remove a lot of material with 40-45W.


One other note, if you are going to worry about it, you might want to be more concerned about fluorine. The mineral Flourite (CaF2) is pretty common in limestones. You can usually check by shining a blacklight on them since it is often fluorescent.

I have no idea if the laser is powerful enough to break the Ca -> F bond though, flourine is pretty active stuff!


That’s good to know, @jkopel. There’s a Flourite mine in the town I live in and I have a bunch of pieces. I had thought about trying to engrave it but I’ll skip that.

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@jkopel do you know which types of rocks would melt to black glass? I posted a thread about this but I only know by testing which ones work for me. Thanks!

That was not something I ever thought about when collecting, so I am afraid not.
I think you are on the right track with experimenting though.
You might want to try some and then bring the ones that work well to your local college/university and see if someone can help you identify them, or maybe there is a rock/mineral collecting club near you?

Sounds like a plan :slight_smile:

Geo here,
I would stick to silicate type rocks for etching due to them having the appropriate chemistry for melting. Specifically, I imagine fine-grained volcanic rocks will lend themselves well to engraving.
I don’t think rock salts would etch well based on the chemistry and other rocks (ie, random pebble from the river bed) with minor amounts of chlorine are likely low enough concentration to be of much concern.
I will be experimenting with this a lot once my GF arrives and I will be glad to post my findings.


Looking forward to your results!

With the proliferation of Himalayan pink salt slabs in cooking, I would imagine the question of engraving might come up. There are a couple of Q&As about this in various laser forums. What gets released in the laser interactions is important. With both sodium and chlorine rather nasty stuff in the elemental form, the interaction with the parts of the Glowforge by volatiles is a good question. I don’t have an answer, and having some geological and minerological resource on the forum would be helpful.

I am not a chemist (nor do I have anything beyond first year college chemistry), but I have google. :wink:

I think the important question in the case of NaCl is one of inorganic chemistry.
As in, it is very different than releasing the chlorine from PVC since the bond is so strong.
Solid NaCl melts at 1474 °F, and boils at 2,575 °F, but I was unable to find any reference that says it ever turns back into Na and Cl.

I don’t know for sure, but I would guess that the likely effect of hitting solid salt with a 40-45W laser would be to blast out tiny pieces of salt without breaking it down to its elemental constituents. While the salt dust might be bad for the Glowforge, it is a lot safer to breath.

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Good point to consider. What is the temp of the beam interaction with the material of a 40-45W laser? A function of time and area of dispersal (focused spot)? Looking into this further, I noted that there is such a thing as optical quality NaCl that is used for laser lenses. The more you know.

I’ve melted salt with my Freznel lens and it just became salt again when I stopped hitting it.

Well, if NaCl is used as a lens, that means it has very low absorbtion. And to answer the temperature question, you need to know the absorption of the material in question. So with low absorption, it would mean low temperature.

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