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Sand-Blasting Glass

sand blasting, etching, glass, shot-blast, cleaning, restoration, cleaning

Briefly, Sand-Blasting consists of blowing an abrasive at a surface using compressed air. Where the abrasive hits glass it roughens, or eventually erodes, the surface.
Sand-Blasting is used on many different materials for many different purposes. The uses range from cleaning the old paint from the Golden Gate bridge to carving on gem stones. Here we'll just cover blasting on glass. The two main effects people are after are etching or engraving, which is a surface effect, and sand-carving where the glass is actually carved away.
Etching glass, or engraving it, refers to a technique where the glass is simply roughened to produce a translucent "white" area on the surface. This effect is produced by lightly blasting the surface with an abrasive or using hydrofluoric acid. The results can be a simple monotone design on the surface, or it can be a near-photographic rendering of a scene in shades of black and white.
You can control the shading with a half-tone photo mask or by very careful use of your blaster, with varying angles, air pressure and selective masking.
Sand-carving is the term often used to describe deep multi-level abrasive etching. The design is carved in the glass and the edges of the different "stages" can create additional detail with light and shadow. Edge lighting can produce spectacular effects.
Whether you are etching or carving, the first step is to pick out a design that reflects the limits and capabilities of how you plan to blast the piece. Surface etching requires a simple "black and white" block design. Shaded or multi-stage carving allows more complexity.
Once you have decided on a design, you need to construct a mask to prevent the abrasive (or acid) from etching the "clear" areas. At this point you need to make a choice between a hand-cut mask or a photo mask. For a hand-cut mask, the design is transferred to the "resist" or mask material. Usually this is by either drawing directly on the resist or with carbon paper. The mask is cut with a knife, stuck to the glass and the removed from the areas to be blasted. In the case of multi-stage carving, you need to plan on peeling away the mask in pieces as you work on each different depth of carving.
For a photo mask, a positive copy of the design is created on a laser printer or a copier, the sheet of mask material is exposed, developed and glued to the glass. You then proceed to etch or carve the material. You can etch mechanically, using acid or Sand-Blasting. Carving is only possible with Sand-Blasting. Read on for more details on all of the above. Good luck, watch the safety precautions and have fun!
Acid etching
Using hydrofluoric acid to roughen the surface of the glass requires much less equipment than blasting. It is simple, and can use the same masks and resists. Used carefully and correctly, the materials can be used safely by adults. However the acid used is dangerous. This is NOT an activity to share with your children. The effects of the acid are not the burning associated with other strong acids, but it can travel through your skin painlessly and attack bone. Rubber gloves and eye protection are a must.
The acid usually comes as a paste. Mask away the areas you don't want etched, apply the paste and wait a few minutes. The paste can then be safely washed down the sink and you are left with an etched surface. You can't use the technique for 3D carving, but it is a quick and easy way for marking or decorating the surface of flat pieces. Using it on curved surfaces is possible, but difficult. A related product is designed to selectively remove the "silver" from mirrors. A variety of prepared designs specifically for acid etching are available. Eastern Art Glass and Warner-Crivellaro, among others, offers a selection of them.
Other techniques
Engraving glass once meant scratching or stippling a pattern in the glass with a diamond. Diamond point engraving of glass by hand is actually being revived as a decorating technique, but it is very demanding art form. What most refer to as engraving today is done by a computer driven diamond burnishing machine. This £2,000 to £25,000 machine looks like a cross between a plotter and a router. A small rotating diamond bit scratches a design on the surface of brass, plastic or glass. A special jig holds wine glasses or bottles and allows engraving of curved surfaces. These are used mainly for the trophies and awards industry.
Note: the laser engravers that can be used in place of the mechanical engravers do NOT work well on glass. Some manufacturers have retracted their claims about this technique and it is rapidly falling out of favour. The stresses introduced into the glass by intensely heating the glass in a small spot can cause problems weeks and months later.
Wheel cutting is the same technique used on your grandmother's cut glass or Waterford crystal. Abrasive wheels, or diamond dust on copper wheels, are used to cut away the glass in the desired design. Lead crystal, which is softer than most glass, is most often used. The higher refractive index of the crystal also shows off the cutting better.
Sand-Blasting - Equipment
the list of equipment needed is short but expensive:
£10 to £800 Blaster
£200 to £2000 Cabinet
£300 to £1000 Compressor
£40 to £400 Dust collector
£20 to £200 Abrasive
Some of the suppliers sell complete systems with all of the necessary equipment and supplies. The minimum system available is something like the Mini-blaster plastic tabletop unit from Photobrasive. Using an existing shop vac as the dust collector, you can blast small (8 inch) pieces for under £300 plus a compressor. A bigger unit is Glastar's Mini-passer for £775 with a siphon blaster. This free-standing steel cabinet lets you work on 12 inch 3D pieces or glass sheets up to 24 inches wide. Again, you need to add a dust collector and compressor.
Blasters
There are two types of blasting "guns", siphon feed and pressure feed. The siphon feed guns work like an atomizer or an airbrush. A stream of air literally sucks up the abrasive and the combined stream of air and "sand" blasts out a fairly large nozzle (up to 1/4 inch). The "gun" can be inexpensive (£10 and up). This is what you see at Sears or you local tool discounter. Note that many siphon guns don't have replaceable nozzles. Carbide will tear them up in a matter of hours. Pro: Cheap, readily available. Cons: Lack of fine control (1/4 inch nozzle), can easily clog. Use lots of air (up to 5 times more). The second hose (for the abrasive) can be awkward.
Pressure feed blasters use a pressure pot (a tank) to hold the abrasive. The air supply then pressurizes the pot. A stream of air and abrasive travel through the same hose to a "gun" that is typically just a small hand-held nozzle (3/32 to 1/16 inch). The flow is best controlled by a foot switch. A pressure pot is relatively expensive - £400 to £800 for a 40 lb. to 100 lb. unit including the foot switch. Pro: More precise control, smoother flow, much less air needed (smaller, cheaper compressor). Only one hose to blaster nozzle. Cons: More expensive. If you don't have a really good water trap in the air line, abrasive can cake up in pot.
One alternative to a big and expensive Sand-Blasting set-up is a mini blaster. They are very limited in what they can do, but are far, far less expensive. There are actually several different products that are called "mini blasters".
Paasche sells a modified air brush called an "Air Eraser" that dispenses very fine abrasive instead of paint. It can be used with an airbrush compressor. It was originally intended for removing ink from paper, but can be used for a little gentle abrasion on glass. Good only for small areas of light etching. Some type of enclosure is needed. The plastic desk top unit from Photobrasive, or a similar unit available from Eastern Art Glass, is sufficient.
Pencil blasters or bottle blasters are small units that resemble miniature pressure pots. The names come from the small, pencil shaped nozzle connected to a bottle sized pressure tank. Again, they require an enclosure and are only suited for small, light etching tasks. They also need a compressor, but a much smaller than one a full sized blaster. Typically 1 to 2 CFM at 80 or 90 PSI is typical. A large air brush compressor may be capable of that. Eastern Art Glass has one unit holding 1 pound of abrasive, for £90. It produces a pattern 1/8 inch in diameter.
Cabinets
Two typical small cabinets are mentioned above. Glastar, Photo-brasive and Rayzist all sell a wide variety of cabinets. Call and ask for a catalogue. When picking a cabinet you need to plan on the size and kind of pieces you want to blast. The cabinets have different means of opening - top, side, big doors and little ones. Also some have the capability to slide a sheet of glass through slots in the sides of the cabinet, allowing you to blast large sheets of glass in a relatively small cabinet.
Blast rooms
When you are doing big pieces, there is no substitute for a room dedicated to Sand-Blasting. You need a hood designed for blasting and a blower to feed breathing air to the operator. Trying to breath the air in the room, even with a dust mask, doesn't work. You also need a REAL big dust collector. For an occasional big project you can create a blast room using plastic sheets that run from ceiling to floor. But don't try to cheat on the air supply.
Doing it in the road...
Some craftsmen do all of their blasting outside, next to the garage or in the alley out back. It saves the space and expense of a cabinet or blast room. This won't work in Minnesota in January, and it won't work in the 'burbs since abrasive goes flying everywhere and most neighbors consider that a nuisance. This is the one occasion where fine sand (80-100 grit) is an acceptable abrasive since you can't reuse the abrasive. But even outside garnet cuts faster and doesn't have the health problems. But in either case do wear a good dust mask.
Compressors
You can get by with a £10 siphon gun, a cardboard or plywood cabinet and an old shop vac for dust collection. But there just isn't any way out of getting a good (read: expensive) compressor. And you need a bigger one with a siphon gun since it uses so much more air than a pressure pot to do the same work.
To do the full range of Sand-Blasting you need a minimum of a 3.5 horsepower compressor with a 20 gallon tank. That will usually keep up with a pressure pot with a 1/16 nozzle. With that, and a siphon gun, you will still have to stop and allow the compressor to catch up with you. A 5 horse unit, with a 60 gallon tank, is better for either type of blaster. But that is more than you can support on a 110 volt electric circuit. And any compressor is expensive and noisy. Check at Home Depot or a similar store for a selection and typical prices. Happy compromise.
Dust collector
Sand-Blasting creates lots of very fine dust that has to be collected or you won't be able to see as you work. The cheapest solution is a shop vacuum, but they are noisy and don't do a good job of catching the finer dust. If you use one, add the optional bag and try to send the exhaust outside. Most of then let you add another hose to the exhaust side to direct the air out a door or window.
The professional units use dust collectors reputedly designed for Sand-Blasting. They are very quiet and are efficient at collecting all of the dust. But they have very short service lives. Plan on replacing the motors (£40) every 4 to 6 months if you use it every day.
Abrasives
Sand is cheap, but it is rarely used as an abrasive for glass. It is fine for removing paint from the Golden Gate bridge, but it is rather coarse for glass work and the silica dust it releases is considered a health hazard. Sand normally comes in 30 grit, but for glass blasting a 100 to 180 grit abrasive is what you want. Pick one of the other abrasives below.
Garnet is widely available and fairly inexpensive (£25 for 100 pounds in California). It looks like purple sugar. It is not as hard or fast cutting as carbide or aluminium oxide but far less expensive initially. It is a good choice for a total loss system (doing it in the road...). Look for it at companies that rent industrial Sand-Blasting equipment. Clementina is one national chain that sells it.
Aluminium oxide is next up on the hardness scale and price scale (£40 to £50 per 50 pound bag). It has one disadvantage. It picks up a static charge when being used and tends to cling to the glass. That can make things hard to see. The higher grade (i.e. more expensive) white variety is of no advantage. Get the cheaper brown variety. Buy it from the Sand-Blasting suppliers below, or look in the phone book under Sand-Blasting or abrasives.
Silicon carbide is the ultimate in price and hardness. It costs £1.40 to £3.00 per pound in 100 pound bags, but some maths shows it may be the cheapest to use since it can be reused more times. As you use an abrasive it slowly breaks down into smaller particles. The smallest disappear into the dust collector. The bigger particles round off and become slower cutting. The carbide grains keep their sharp edges when they break up and stay more effective. Carbide cuts faster and easier than the other abrasives. The common black variety is fine for working with glass. Buy it from the same sources as aluminium oxide.
Resists
Resist is the mask used to keep the abrasive or acid off the areas you don't want blasted. Resists come in two types, hand cut and photo resists.
Hand cut masks are made by literally cutting the design out of a piece of material that will resist the abrasive long enough to get the effect you want on the glass. You can draw on the resist or transfer the design with carbon paper. My favourite is to print the design life size from my Mac - no scaling problems here! Then hit the back side of the paper with Spray Mount glue, stick it directly on the resist and cut through both to create the mask. What ever technique you use, buy your knife blades in the large economy size package. You will need to keep a perfectly sharp blade in the knife at all times or the mask will stretch and distort as you cut it.
Everyone starts out using Contac brand paper for their first hand cut masks. It's cheap, but it has its problems. The glue is intended to be permanent and it often takes several layers to allow deep cutting. By then it costs as much as vinyl resists. Contac paper does have the benefit of being easily available. Use the foam cushioned variety for deeper cuts. It lasts a little longer and costs about the same.
Several companies make vinyl and rubber resists intended for Sand-Blasting. These cost more than Contac, but you get a better product for your money. They vary in thickness from 5 mil for preventing scratches on areas you don't want blasted to 1/2 inch for carving granite tombstones. The 8 to 25 mil varieties from 3M, Anchor Continental and others cut easily and work well on glass. To find Sand-Blasting resists, check in your favourite glass magazine for ads, ask at a local stained glass shop or check the suppliers below. Another possible source is a commercial glass shop that advertises Sand-Blasting work.
Photo resist material creates the mask by a process similar to a photographic contact print . You can handle the unexposed resist in dim or red to yellow light. Print a full sized copy of your design on a laser printer or high quality copier. You need to print on acetate, like for an overhead projector, or vellum. (Note: If you have an ink jet printer, make a copy of your art work onto vellum or acetate with a high quality copier. The original ink jet printing is not UV opaque.) The design can be simple black and white or with some resists it can be as fine as a 35 screen half-tone (that's about as detailed as a crude newspaper photo). Place the original against the resist and expose it to UV or bright sunlight. The exposed areas toughen up. You then wash away the softer unexposed areas with a stream of high pressure water. You are left with a mask to glue onto the glass, pull off the plastic carrier sheet and blast away. DuPont has come out with a new material in 1997 which skips the washing stage. A liquid emulsion is also available for use on flat surfaces. Photo resist sources include Cooper Graphics, Photo-brasive and Rayzist . The latter two sell UV exposure systems starting at about £225. For occasional use you can use the sun or make a light box with black light bulbs. The photo resist material runs £0.05 to £0.12 per square inch.
The same companies that produce the photo resist materials will take your artwork and produce the masks for you. There are also catalogues of prepared masks for both blasting and acid etching.
Suppliers
Below are some of the many suppliers. Eastern Art Glass and Warner-Crivellaro are mail order hobby suppliers. The others are industrial suppliers, not retail outlets. So expect only box lot sales and minimums of £25 to £100. Check with your local glass shops for other sources. If you have recommendations based on personal experience, please let me know at info@sand-blasting-uk.com

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