Spray Glazing

The guidelines below are recommended best practices for spray glazing, and for using the studio spray booth. The booth is 36 inches wide by 48 inches tall by 24 inches deep, and is located to the left rear of the glaze room, behind the bisqueware storage rack. It is connected to a powerful exhaust duct, and has a dedicated air compressor to operate standard spray guns.

Figure 1. The Studio Spray Booth

Figure 1. The Studio Spray Booth

Air Duct Set Up:
Our clay mixer and the spray booth both share the same exhaust fan, so one fixture has to be blocked whenever the other is in use. Normally you should expect to find the spray booth exhaust blocked because it’s used less frequently. Begin by slipping the cardboard at the back of the booth underneath the wire holding it up, shown left in Figure 1, and place it to the side of the booth. That opens the exhaust duct (Figure 1, Right)  so that air and glaze overspray can be readily vacuumed out the back. Note that the cardboard needs to be replaced after glazing is finished.
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Next, turn ON the fan switch located to the right of the booth. You’ll hear the fan motor start, and air will begin flowing through the booth and up/out the exhaust duct.

Mixer Vent

Figure 2. Clay Mixer Exhaust Duct

Then, go into the kiln room and place a 2′ x 2′ cardboard up close to the clay mixer exhaust duct shown in Figure 2. Let fan suction pull it up to seal the duct.  While it’s in place, suction through the spray booth increases to its best level. After you finish spraying, remove the cardboard and also store it at the side of the booth.
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Figure 3. Booth Set Up

Ware Set Up:
Place a Shimpo banding wheel with a tall base onto an empty plastic bucket so that your piece is elevated and close in front of the suction area – most importantly, away from the metal booth walls. The banding wheel will let you slowly rotate the piece while spraying so that all its outside surfaces can be exposed to the spray. The piece should have a waxed foot done in the normal manner. Figure 3 shows a typical piece ready for glazing. 

Next, check out the key elements of the air compressor that are identified in Figure 4.  Click to enlarge. The compressor is located on the floor to the right of the booth.

Comp

Figure 4. Compressor Set-Up

Compressor Set Up:
Connect either your gun or a studio gun to the Air Output Connector, and fill the gun reservoir with a little clean water. Turn the Compressor Switch to the  “Auto”  position.
Tank Pressure will increase to about 150 PSI.  The Output Pressure should be much lower: 35  to 50 PSI.   Adjust up (+) or down (-) with the Output Pressure Adjustment Knob. You can go as low as 20 psi; glaze comes out more gently and with less blow-by as pressure is decreased.

Gun

Figure 5. Spray Gun Preparation

Spray Gun Preparation:
Test gun operation with clean water first. Spray guns normally have several adjustments to alter glaze volume and spray pattern.  Poor, spitting, or erratic spray indicates adjustments are needed to the gun, glaze clogging, unclean or worn gun parts, or too low output pressure. Test, adjust, and clean as needed. 
Recommendation: Clean with monofilament or other soft, but stiff materials – not pins or hard metal items.
Guns generally spray in an elliptical pattern.  The long axis of the ellipse will be perpendicular to the orientation of the spray head.  In Figure 5, the spray ellipse will be horizontal.  Generally you want to spray vertically for tall pieces, and horizontally for shallow pieces and for rims.

When you’re comfortable with the spray volume, coverage, pattern, and pressure, empty out the water and fill the reservoir with the desired glaze. Test again and tweak settings if needed.

Spraying Techniques:
Recommendation: Wear a respirator or mask when spraying. Also, consider wearing safety glasses with side shields to keep spray out of your eyes; noting that some minerals are toxic.

Any paint sprayer will generally handle all of our glazes and stains, however some of the thicker slips do need to be either diluted or defloculated.  When using slips, adjust the specific gravity to about the same as a glaze (1.3 to 1.6), or defloculate (See Defloculation) to an equivalent pourability. 

Hold the gun about a foot from the piece. Then, spray a quick burst at the side of booth as a final check that the glaze is coming out nicely.

Turn the banding wheel very, very slowly.  Spraying inside or outside first has most to do with how much handling you will have to do and how many different effects you want on which surface.  Bowls are often best sprayed outside first by putting the piece on its rim so you can clean the foot by holding the unsprayed inside.   You will need to think about the approach that will cause the least handling.  Spray awkward and hardest-to-reach areas first. Be careful when spraying the inside of a bowl; the glaze can circle around and blow back at your face.
When spraying tall cylinders, be careful as the shape of the piece can force spray into your face (thus the need to wear safety glasses).

Spray Thickness:
Glaze surface and color duplicates a dipping application if the glaze is applied similarly thick and dense. Generally that means you need to put 1 to 3 mm of spray onto the piece.  This can be tested with a pin marked with mm on it.  However, thickness and density are not as related when spraying when compared to dipping because spray deposits have more air in them depending on the gun distance.  Moving the gun closer to the piece produces wetter, denser deposits; moving it farther away leads to lighter, fluffy deposits.

Mark the banding wheel so that you know when a complete revolution has been completed. Consider the effects of tapering the spray both horizontally and vertically.
Also, consider tapering the spray so that there is less glaze near the foot.

Waxed Decorations / Multiple Glazes:
If you have created waxed decorations, notice that spraying tends to run off the wax and leave drips below the pattern. Consider scraping the drips with a razor as needed.
Different glazes can be layered in different thicknesses allowing an infinite variation in color, density, mixing, etc.

Taping and Stencils:
Spraying allows for complex designs that are much more difficult to do with dipping. For example, tape can be put directly onto the bisqueware to make lines.  The piece can be sprayed and then the tape removed and a new glaze sprayed onto the piece or tapered into the previously taped area.
However, avoid taping already sprayed areas, EVEN WHEN DRY.
Stencils may be applied on the piece and sprayed over.  Be careful not to compress the base glaze.

.Handling After Spraying
Let the piece fully dry in the booth. Be very careful handling it afterward because the spray has lots of air in it.  Touching compresses the glaze and causes some of it to either fall off or be left on your hands. Also, sprayed glazes don’t tolerate over-glazing with a brush very well. 

Booth Cleanup:
…….1. Sponge down the booth thoroughly so that remnant dried glaze doesn’t turn
………..into dust in the air. Leave the booth in a cleaner condition than you found it.

…….2. Turn off the exhaust fan, return both cardboards to their original location.

…….3. Turn the compressor switch to OFF.

Gun Cleanup:
Remove the reservoir and pour remaining glaze into glaze bucket.  Rinse the can thoroughly.  Remove the nozzle section of the gun.  It screws off counter-clockwise.  Rinse thoroughly.  Wash the gun head by running water through the suction tube and holding the trigger open.  Pull the trigger multiple times as you do this until the water runs clear, or fill the can with water and spray into a sponge until it runs clean.
Dry the gun off as it has some parts that will rust. 
After the gun is dry, spray the adjustment knobs and the spray nozzle with WD40.  Pull the trigger to make sure all the moving parts are lubed.  Lightly wipe off. 

Store the studio’s gun in the gray cabinet.
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