I think this would make a very interesting thread. Let's go for
it !!! MTR
Mark (And any other interested parties)
Hope this isn’t a let-down, after waiting so long
Okay. (Deep breath) To begin, I am by no means an expert on
sandblasting letters in stone. I am more comfortable with
regular carving methods (give me a diamond burr and stand back).
I recently aquired a rather nice micro-sandblaster. The idea was
that it would help me in the interim stages of carving (sanding
and prepolish). I didn’t hold out much hope for it being
capable of doing the actual carving.
This blaster was designed for the computer and electronics
industry and one of its sales pitches was that it could drill
holes easily through ceramic. Not having any ceramic handy, I
tried it out on an agate formation and incredibly, it cut through
not only the crusty mineral coating, but through the agate
itself! (It’s kinda weird seeing agate “melt away” like an ice
cube with hot water running on it).
Now before you run right out and get yourself a new sandblaster,
let me tell you about my current project. I am trying to do
some cleaning and prep work on a very beautiful, very large
crystal section of aquamarine (Alas, its not mine, but belongs to
another member of the Orchid list). This crystal has some calcite
formations on it that the blaster has no problems with. Then
there is a “zone” where the calcite and beryl seem to have
blended together (pseudomorphed?). The blaster has a harder time
with this the deeper you go (the closer you get to pure beryl).
If you make it to the beryl itself (mostly, I’m not), you could
park that nozzle there for 10 minutes and the best you can do is
turn it frosty. This is using 120 grit aluminum oxide at 90 psi
(about the maximum size and pressure suggested by the
manufacturer). My theory as to why it works so well on agate (7
mohs har edia to “get a foot hold” and break up the stone,
whereas the beryl’s more structured crystal formation acts as a
rigid barricade. - again, this is just my theory.
I talked to a local monument (gravestone) cutter about the
processes they use for cutting letters. They have more than a
few ways of doing it. As far as sandblasting goes, this is my
understanding of the process: They take an already polished slab
of stone and cement (I’m assuming rubber cement) a rubber stencil
on the face of it. I believe he told be the rubber was a butyl
rubber, but am unsure now. The slab is placed in a blast cabinet
and the switch is thrown. Their blast machinery is very high
powered and I believe he uses AlOx as his media. This process
makes very crisp outlines for the letters, but the sides and
bottom of the cut are not very well defined.
While I’m sure I’m not a pioneer in this area of lapidary, I feel
as though I am. I have been unable to find any published
material on the subject (Vicki Embry - Thanks for the info about
those Lapidary Journal articles! I’m looking into it ASAP). The
manufacturers of the blaster aren’t a lot of help, as they don’t
know much about rocks. I’ve talked to a few people who use
blasting for fossil and specimen preparation, but even they don’t
ask their machines to do as much as I’m trying.
A few “nuts and bolts”
My particular blaster has a variety of nozzles available with
orifices from .007" dia. to .060" in the round shape, and some
slotted ones .006" x .020" up to .013" x .150".
Grit sizes from sub-micron up to 120 grit (102 micron). The media
I mostly use is either AlOx or silicon carbide. I don’t notice
much difference between the two - SiC is a little harder and
AlOx is a little denser (hits with greater impact), so its kind
of a tie for effectiveness. As for polishing: Most of the
things I’ve worked on to date were such that I could use regular
polishing methods (felt wheels with diamond on my foredom unit).
On the aquamarine, I am going to try glass beads for a near
polish effect (My client’s suggestion). I’ll let you know how
it works out when I’m done.
Masking (or resist): Again, I haven’t needed to use this process
yet. For this article, I did try the blaster on a few
materials. My conclusion is that the more “bounce”, the better.
I have a few scraps of various types of rubber around. The
flimsier, bouncier stuff held up under the blast much better
than the harder, tougher stuff. My guess is that good old
office supply rubber cement or silicone window seal (The clear,
rubbery stuff) would both work well. Thickness of the
application would depend on the amount of time under the blast.
My blaster, being of the micro variety, is easily controlled, so
I haven’t needed to mask yet. The drawback to this is that it
doesn’t cover a lot of area at once.
Maintenance: This is an area that the manufacturers don’t tell
you much about until after you purchase your machinery. Of
course, when you think about it, anything that is going to break
up rock when it hits, is also going to play havoc on supply
hoses, nozzles, fittings, etc. On my machine, the hose running
directly off of the media/air mix tank blows out and needs
replacing about every 5 - 10 hours of operating time. If you
get a kink or tight turn in this hose, it blows out in seconds.
Nozzles ($8 - $15, US ea.) generally last about 50 hrs, I’m
told. These are tungsten carbide nozzles. They also make
sapphire nozzles (about 2 - 3 times as much cost). I don’t know
if it’s worth the extra cost at this time. There are a lot of
other various couplings, fittings, etc. that require regular
Conclusion: There are so many variables involved in this question
of cutting letters in rock. Stone hardness/toughness; The
"look" you want in your letters (If you want crisp, well
polished cuts, a blaster probably isn’t the way to go);
production amounts and size of letters; The equipment you have
available for use (I’m sure a full-size blaster with a constant
air supply of 80 - 120 psi would be able to cut much faster.
Then you would need to worry more about the masking or resist
problem. You would also need a very large, very powerful and
expensive air compressor).
In short: Blasting is not the tool for cutting letters in
stone. It is one of many tools, depending on the application.
I personally would use a combination of tools to carve letters
(electroplated or sintered diamond burs and blades to cut;
sandblaster to sand and prepolish; felt wheels and diamond or
oxides to polish) . If anyone has a specific application in
mind, I’d be happy to look into it and give advice and/or
I hope this is of some use to you all. I am keeping many and
detailed records of all my experiences with blasting. Someday,
I may even try to “tie them all up” and make them
understandeable to anyone aside from me.
Your resident "tool freak,"
Stone Broke Custom Lapidary