Introduction to Sandcarving – an overview

Author: Tony Smith

Posted:

Revised: August 03, 2005

Publisher's note: Tony has been kind enough to generate this overview document, based on his experience and point of view. It's an excellent article, and not easy to do, since each of us approaches our work from a different perspective. It's important that we give beginners a broad and consistent view of sandcarving. If you have additional thoughts or information that should be included, please contact Tony or Jim Yount.

Sandblasting can be used as an accent to your stained glass or fused glass, or it can be the major aspect of your glasswork. With an understanding of the basics and a little creativity, this technical toolset can help create reproducible effects that are otherwise impossible to create.

Sandblasting Materials and Methods

Most materials be sandblasted except for diamond which is harder than all sandblasting media. Stone is regularly sandblasted as a way to create gravestones and memorials. Metal is sandblasted to remove rust and sharp edges or as a way to enhance adhesion of coatings and adhesives. Wood and brick is sandblasted to remove years of paint, oil and dirt and to produce a clean surface for staining and painting. Certain types of plastic can be sandblasted for decorative effects, but the work has to be done very carefully, as the plastic can be much softer than the abrasive and will wear away quickly.

By mixing air under pressure with a hard, particulate abrasive or media, material can be slowly abraded or worn away by directing the abrasive stream onto the surface of the material. The longer the abrasive stream is held in one spot, the more material will be removed. Moving the nozzle closer to the surface decreases the area being blasted, and increases the rate of material removal. By moving further away, the area increases and the rate of removal decreases. If the nozzle is held very close to the material, a hole can be abraded through the surface very quickly.

Glasswork

In glasswork, sandblasting can used to etch a pattern into the glass surface, creating a frosted effect. A three-dimensional design can be carved into the glass volume by using a “stage-carving” technique where some parts of an image or pattern are carved deep into the glass while others are carved shallow or only surface etched. Sandblasting can also be used to selectively etch the surface of the glass to remove iridescent coating, enamel or micas. Also, sandblasting can also be used to remove kiln wash or fiber paper that has stuck to the glass surface during fusing.

Sandblasting can be used to decorate ordinary objects, such as bowls, bottles, glasses, vases or candleholders. Artwork and lettering can be etched into glass windows for decoration or identification. Sandblasting can also used to customize awards by etching names and other information into glass or metal objects. Mirror can be etched from the back, removing the mirror coating to create interesting effects.

Conventional "cut resist" methods"

The first step in sandblasting is obtaining a pattern or design. A two-dimensional design can be as simple as a heart shape, stars or some lettering. A rubber or vinyl sheet material is applied to the item that is to be blasted. The sheet material, which is referred to as "sandblast resist", is applied to the item to “resist” the effect of the abrasive stream.

The pattern or design is then transferred to the resist material with ink, pencil or carbon paper or by “pouncing”. By cutting through and removing selected portions of the pattern, the unprotected surface of the item will be exposed to the abrasive stream where material will be etched or carved away. This is called a positive etch, where only selected areas representing the pattern are etched. An alternate etch, known as a negative etch, can be achieved by leaving the resist on the pattern and removing the background areas.

Photoresist methods

Recently, ultra violet sensitive film technology has been applied to glass carving, with the result that very precise resists can be created using a photographic process. Lines as fine as 0.003 inches (0.20 point), lettering as small as 3 point type, and halftone photographs can be produced on glass and other materials. The process lends itself well to multistage carving, in depths to 1/4 inch and greater, depending on the size of the element being carved. The original line art can be hand drawn, but there are significant advantages to using a computer in the process: line weights can be carefully controlled, to ensure a successful resist.

Basically, the process involves:

generating the line art,
creating a positive film image of the art,
exposing the resist to ultraviolet light,
washing the resist to create open areas through which the abrasive will pass,
applying the resist to the glass
and blasting the glass.

For multistage work, the individual design elements are "cut" by the abrasive on the first blast, then removed in sequence as needed.

The advantages of photoresist are the incredible detail possible with this method, and the elimination of the tedious process of hand cutting the resist. Disadvantages include the addition of computer and photographic technologies to the carving process, and the high cost (five cents to nine cents a square inch) of the photoresist. As the size of the work grows beyond several feet in each direction, the degree of difficulty and cost of the materials grows significantly.

A separate article on photoresist techniques can be viewed here.

Complex images

Complicated images can be etched onto the surface of glass, but care must be taken to ensure that adjacent pattern sections are separated by unetched areas. Another method would be to allow the adjacent pattern areas to touch and to shade the different areas by having a lighter etch in some areas and a heavier etch in other areas. This technique is difficult to master and requires a good artistic sense of black and white shading.

By selectively removing pieces of a pattern and blasting different areas to different depths, a three-dimensional shape can be carved into the glass. This is commonly referred to as glass carving or sandcarving.

Techniques have to be developed by the glass artist to deal with adjacent and overlapping areas in a pattern. These are typically done by numbering the pattern sections by depth and selecting the deepest pattern sections to carve first. The glass artist must stay away from areas that have already been carved to preserve the detail in that section. By carving the glass in stages, intricate sculptures can be created in a two-dimensional piece of glass.

By creating a two-dimensional etch on the backside of a mirror, the pattern can be developed where certain areas of the mirror coating have been removed, and other areas remain. The etched surface can be left frosted, or can be back painted to add some color. Back painting has an interesting effect in that it appears that the color is embedded in the glass.

Blasting

Once the resist has been applied to the glass, the next step is to pressurize the pressure pot or connect the siphon blaster to the compressor and blast the glass, normally from a distance of about 2 to 6 inches away from the surface. Using a painting motion, and overlapping areas, a uniform etch can be obtained. When blasting to different depths, selected areas of resist are removed and as opposed to etching, where you try to get a uniform frost over the entire piece, the sandblasting is concentrated on small areas of the item. The edges of the resist are traced to obtain a sharp edge and the open areas can be carved deep. Special techniques are used to deal with overlapping sections such as twisted vines or leaves. Sections of resist covering successively shallower areas are removed and the blasting continues until all sections have been peeled and carved.

After etching or carving, the item is rinsed off under water, being careful not to drag any grit across the surface of the glass. A single particle of silicon carbide can ruin a glass surface or the back of a mirror because it is much harder than glass and will make a very deep (and very obvious) scratch in the surface, so it is very important to ensure that all media had been washed off. You also do not want to transport any grit to your workbench where it could scratch a piece of glass in the future.

After washing and drying, the entire piece should be inspected to see if any areas were missed. This is the time to catch any missed areas, because once removed, the cut resist cannot be reapplied. Also, it is very difficult to apply and properly align a cut using new resist on a piece of glass that has been sandblasted.

The resist can now be removed from the glass, and any paint (if desired) applied.

Sandblasting Materials

1) Materials which can be sandblasted:

a) Glass, stone, metals, wood and plastics steel, brass, aluminum, pewter, silver, etc.

2) Materials required for sandblasting:

a) Abrasive Media – Sold by the pound and grit size 80 grit = 0.007” sieve mesh opening; 120 grit = 0.0049” opening; 200 grit = 0.0029” opening

i) Sand – Single use. Lowest cost at <$0.05/lb. Produces free silica: cause of silicosis, a lung disease

ii) Aluminum Oxide – Lasts 30-40 x over sand. Creates static charge. Moderate cost at $1.00/lb

iii) Silicon Carbide – Lasts at least 40-50 x over sand. Highest cost at $1.60/lb Cuts faster than AO, wears hoses and nozzles faster, has no static charge, and fractures leaving sharp edges vs. rounding of AO.

iv) Garnet

v) Glass Beads

vi) Black Magic

vii) Walnut Shells

viii) Plastic Pellets

b) Media screen – To filter glass remnants and resist from media before reuse.

c) Compressor - Supplies compressed air – pressure measured in psi (pounds per square inch) and flow in CFM (cubic feet per minute). Moderate to high cost at $250 - $3,000 and up

d) Media delivery system

i) Pressure Pot – Uses less air volume and lower pressure. 5 to 50 psi. 5 to 9 CFM. Highest cost $300 - $700

ii) Siphon Blaster – Uses high pressure and large volume of air. 35 to 90 psi. 10 to 14 CFM. Low cost ~$100

iii) Handheld Blaster – Holds small amount of media. Inexpensive. Good for infrequent use. Cost ~$30

iv) Recycling Blaster – Pressure pot, nozzle, vacuum all in one unit. Good for site work. Hard to achieve even frost due to small blast area. Cost ~$600

e) Sandblast Nozzle – Measured by inside dimension, type and material. We use Type 2 nozzles, and 3/32”, 7/64” and 1/8” inside diameter nozzles

i) Ceramic – Lasts about 1 hour or less depending on pressure. Low cost ~$3 ea

ii) Carbide – Longer life by 20 x over ceramic. Higher cost ~$35 ea

f) Sandblast Cabinet

i) Plastic – For occasional use and pen blasting. Portable. Relatively inexpensive at $150 - $250

ii) Steel – For regular use. Heavy, stationary. Moderate to high price at $300 - $10,000

iii) Homemade – Wood or plastic. Could be plywood, a Rubbermaid tub, a cardboard box or even a plastic trashbag. Cheap but difficult to use.

g) Dust Collector (vacuum)

i) Shop Vac – Adequate for occasional use – loud – inexpensive. $50 - $100

ii) Specialty Dust Collector – Better filtration – For regular use – 3 x – 10 x cost increase. $250 - $1500

h) Sandblast Resist

i) Contact Paper – Good for very light etch. Blows through, or lifts with higher pressure. Easy to cut. Adhesive sometimes difficult to remove. <$0.25/sq ft

ii) Vinyl Resist – Good for light etch and protecting mirror surface. Easy to cut. Adhesive easy to remove. Moderate Cost at $1/sq ft. Manufactured by Venture Tape and others. Also available for computer plotter cutting.

iii) Rubber Resist – Excellent for deep etch or stage carving. Different thicknesses to accommodate different depth requirements. Easy to cut. Moderately expensive at $1 - $2/sq ft. Manufactured by 3M (buttercut) and Anchor-Continental.

iv) Photo Resist – Different thicknesses for different depths of etch. Requires laser printer or high quality ink jet printer, vellum or film transparency material, ultraviolet exposure unit, and washout unit. Very expensive at $8 - $10/sq ft. Requires practice and experience to avoid significant waste. Provides a level of fine detail that cannot even be approached by other methods, and a method of producing halftoned photographs onto glass and other materials.

v) Found Resist – Stickers, stars, paper doilies, metal objects. Good for light etch. Consider Bob Pickard’s liquid resist paper.

vi) White Glue – Good for odd shapes. Strong resist. Inexpensive. Easy to remove.

vii) Masking tape - Painters tape is the best because it doesn't leave a residue and the adhesive is water soluble, but it is the most expensive at $2 to $4 per roll.

viii) Paper Plates - Good for covering large openings in bowls.

Suppliers

For a list of suppliers, see Section 4 in the sandcarver.org table of contents

Shortcuts and cost savings

Use heavy duty air hose instead of sandblast hose – 10% of the cost (and a fraction of the life).

Use Silicon Carbide media, more expensive initially, but lasts much longer.

Use Quartz Halogen floodlight in cabinet

Cover vacuum port with a plate on standoffs or a bent U-channel of sheet metal to reduce abrasive loss into vacuum. Or use an in-line reclaimer