A method of cleaning or whitening using sand.

Finely ground sand is sprayed at a surface at high speed. This causes any dirt on the surface to be blasted away, or in the case of whitening, the action of the sand scars the surface so that it is lightened.

Sand Blast, one of the most wonderful uses of sand, by means of which glass, stone, metals, or any other hard substance may be cut or engraved. If a stream of sharp sand be let fall from a high box (as high as the ceiling of a room) through a tube on to a plate of glass held under it, the sand will cut away little grains of the glass till at length the whole surface will be cut or scratched and it will look like ground glass. If, instead of cutting the glass all over, it is wanted to engrave a pattern or figure all over, the workman only has to cover the parts of the glass which he does not want cut with a stencil plate made with leather, rubber, paper, wax, etc., for the sand will not cut any soft surface. Metals and stones may also be cut by means of the sand blast, which will not only scratch the surface, but will cut it away to any depth. The marble tombstones put up in the National cemeteries to the memories of soldiers killed in the war were made in this way. Iron letters were fastened on to the smooth face of the stone, which was then put under the sand blast, and the sand cut away all the marble not covered by the letters. when the iron patterns were taken off, the letters were left raised as if they had been cut out with the chisel. The work was done so fast that 300 headstones were made in a day, or as many as 300 men could have done in the same time, working with the hammer and chisel.


Entry from Everybody's Cyclopedia, 1912.

Sandblasting--more accurately called "abrasive blasting"--was patented in 1870 by Benjamin Tlighman. The idea allegedly came when Tlighman noticed the sand-induced frosting of windows during his days as a soldier in the American Civil War. It seems legit, as the effects of wind-borne sand can accumulate with shocking speed, and Tlighman did spend much time in wooden structures within persistent streams of sand and air. His patent application also detailed that sandblasting could bring out the grain in wood.

Tlighman won the Great Medal of Honor at the 40th Exhibition of the American Institute of the City of New York; soon after that the Franklin Institute gave him the Elliot Cresson Medal. Compressed air entered the process in 1904, thanks to Thomas Panghorn's invention, the air compressor. Wheel blasting is the preferred airless method, whereby centrifugal force acts on abrasive media within a rotating drum.

Sandblasting is today a relatively cheap craft to set-up and operate, and an easy one to learn.

 

THINGS

You need an air compressor, compressor hose, sandblaster, blasting media, nozzles, blast hose, and an enclosed space (a tarp enclosure is sufficient, if the work is too big for a sandblasting cabinet). Note that if you are working in a secluded area or are a shitbag you may sandblast in the open air. 

You want an air compressor capable of holding 90-ish psi for a few minutes at a time. 60 gallons is pretty good. If you're buying used, you'll want to look at the motor's oil level (there'll be a little window on the motor somewhere, and you want it at least half full) and listen to it run for a minute or two. You shouldn't hear anything high-pitched.

A sandblaster is a pressurizable tank with three valves: one to accept air from a compressor, and one each to feed air and media into a blast hose. There is no reliable sandblaster. All build condensation on the inside, require frequent servicing, hemorrhage air from every fitting and grommet, and fail to use the bottom 40% of media without being continuously rocked back and forth.  

Blast hose should be mesh-jacketed, even if such can be expensive. Anything else tends to pop with much frequency and in dangerous moments/directions. Ask me how I know!

 

BLASTING MEDIA

This category contains approximately every thing.

Shell grit is the term for powdered walnut or pecan shells. These are for delicate work, like removing grafitti from concrete or putting a finish on aluminum. Synthetic media in this vein include starches, baking soda, and dry ice, for blasting electronic boards. There are also glass beads, a few different kinds of metal shot.

Aluminum oxide is a good engineered media for blasting metal (most sandblasting applications are metal-focused). It's a large improvement over blasting with real sand, because it a) contains no silicates with which to fuck up your lungs and b) is much less prone to pulverization (see a)).

 

SAFETY

Ever held a sandblast stream on a piece of wood? It melts; the growth rings go slowest.

There is the obvious danger to your eyes, but lungs also face significant threat. For example silicosis, if you happen to be using actual sand (don't). Happily, that disorder doesn't show up right away.

More safety measures apply when you're in the same enclosure as the thing you're sandblasting. It's a good idea to wear some kind of hood with clear visor (I've used shrink-wrap secured to a cloth hood with masking tape, a bunch of times in a row). You can buy a set of Tyvek coveralls for $35-ish, and those can be made to last awhile if you're careful. A good way to pressurize your coveralls (thereby keeping the nasties out) and also deliver yourself air is to direct the outflow from a shop vac through an opening in the suit.

You can create this opening with a drawn circle, a razorblade, and a piece of wood cum cutting board. Duct tape the hose from the shop vac through the opening, or, if you're feeling fancy, duct tape a fitting from the shop vac through the opening and attach the hose to that.

You'll also want breathing protection, and sturdy gloves.

 

SETUP

With all valves closed on both air compressor and sandblaster, attach the air hose to the air compressor outlet and sandblaster outlet (in both cases, it's the highest valve). 

Now is a good time to release the condensed water inside the air compressor. You'll find a valve at the bottom; open it slowly, and close it when only clear air hisses out.  Always open valves slowly. Next, open the outlet valve on the compressor.

Sandblasters are usually capped with a grommeted thing that's held in place from beneath by air pressure. Be sure that's free of media, hold it in place pretty firmly, and (slowly) open the outlet valve on the sandblast tank. The hiss will fade as the tank pressurizes. If you left another valve open on the sandblaster it will now become obvious.

 

TECHNIQUE

You can wear a hole through so many things.

Factors requiring your attention are compressor pressure, the ratio of media to air leaving the nozzle, nozzle wear, and ambient humidity.

70 to 100 psi is a good range for cleaning or creating a finish; it's sensible to use more pressure when blasting inner surfaces, particularly convoluted ones. Note that using high pressure at close range smashes apart your media at a much-accelerated rate.

Cleaning a surface is straightforward. To hurry things along, bring your nozzle as close to parallel as you can with your work's surface. A closer nozzle means more concentrated abrasion. 

A finished surface is best accomplished with one part media to nine parts air. This requires interfacing with the sandblast tank, which particularly sucks to do when you're in a Tyvek bunny suit and the sun is perched on the end of your nose. You achieve this ratio by first opening only the air and then opening the sand as slowly as you possibly can. The exiting media should look like a colored shadow.

Make a steady curlicue motion with the nozzle, maintaining distance. That way, you blend as you go. It's generally best to hold the nozzle eight to twelve inches away.

A worn nozzle causes dwindling air pressure and a fluctuating air/media ratio. I like ceramic nozzles because they're cheap and they last awhile.

Humidity makes it all very ugly and very bad.

 

TROUBLESHOOTING

I stress same re: humidity.

If you can get air but no media, moisture has ruined things before the blast hose; if you can get neither air nor media, moisture has ruined things after the blast hose.

If abrasive will not stop coming, the ball valve that feeds air and media into the blast hose has worn out. Close the outlet on the sandblaster and point the nozzle in a safe direction until the pressure is gone. Replace the ball valve. Buy a couple extra.

If you hear a bang and/or feel pressure and heat, a hose has popped.

 

 


 

sources

Experience

Gero Sandblasting. "The history of sandblasting."
http://www.sand-blasting.co/the-history-of-sandblasting/

Wikipedia.  "Abrasive blasting."
http://en.wikipedia.org/wiki/Abrasive_blasting

 

see also
Preventing Silicosis and Deaths from Sandblasting 

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