To spray glaze without air-related defects, you need air dried by a refrigerated dryer, filtered at the compressor outlet, then filtered a second time at the foot of the booth, at a steady pressure of around 2 to 4 bar at the gun. Water in the air causes runs and thick patches, oil causes crawling and craters, and pressure that drops gives an uneven coat. A 4-in-1 compressor such as the K 75 (1.1 m³/min at 8 bar) already has the dryer and filtration built in.
Choosing the compressor for the whole workshop is covered on our page compressor for ceramics and glazing. Here, we look at when compressed air is responsible for a glaze defect, and how to avoid it.
Where does the water in compressed air come from?
Ambient air contains water vapour. The compressor draws it in, compresses it along with the air and sends it, hot, into the network. As it cools in the air receiver and the pipes, this vapour condenses into droplets. A 7.5 kW screw compressor running for a day in a damp workshop can produce several litres of condensate.
Ceramics workshops are often damp: slip, glazes being prepared, greenware drying, booths being washed down. The air drawn in is loaded with water, and the risk of condensation rises. Without a dryer, this water ends up at the gun sooner or later.
Which glaze defects can compressed air cause?
Not every glaze defect comes from the air: glaze preparation, slip density, bisque porosity and firing all play a major part. But some signs should make you look at the compressor.
| Defect found | Possible cause on the air side | Check |
|---|---|---|
| Runs, local thick patches | water droplets sprayed by the gun | drain the air receiver, see if water comes out at the gun |
| Matt or poorly covered areas | water diluting the glaze on the piece | booth filter full of water, dryer switched off |
| Crawling, glaze pulling back to leave bare bisque | oil or grease deposited on the bisque | greasy mark on white paper blown with air |
| Small craters, pinholes | oil or water droplets in the spray | paper test, condition of the filters |
| Uneven coat, orange peel | unstable or too low pressure at the gun | pressure gauge at the gun while spraying |
| Embedded dust | unfiltered air or dirty blow gun | particle filter, clean hose |
The simplest test: blow for a few seconds onto a sheet of white paper or a clean mirror. A damp mark signals water, a greasy ring signals oil. Repeat it after every change to the network.
What air treatment for glazing?
Treatment is done at three levels, from the compressor to the gun:
- At the compressor outlet, a refrigerated dryer. It cools the air to condense the water and drain it before the network. The air leaving it no longer condenses as long as the network stays warmer than the dew point. Our guide to the dew point explains this idea.
- After the dryer, filtration of particles and oil aerosols. Our compressors are lubricated: it is this filtration that holds back the oil carried over.
- At the foot of each booth, a fine filter, then the regulator. It stops anything the network may have added: rust, residual droplets, dust.
For a particularly sensitive glaze, our technicians can define additional filtration, for example activated carbon, and the air class to aim for under ISO 8573-1. Our guide to ISO 8573-1 air quality explains how to read these classes.
Which compressor for dry air at the booth?
The simplest option is a 4-in-1 compressor, which brings together the compressor, air receiver, refrigerated dryer and filtration. Sizing follows the consumption of the spray guns.
Example of a workshop with three glazing booths and one blow gun (indicative figures, to be checked in the guns' manuals):
- 3 spray guns at 250 l/min, continuous: 750 l/min
- 1 blow gun at 150 l/min, counted at 50%: 75 l/min
- total: 825 l/min
- with 25% safety: 825 × 1.25 = 1,031 l/min
- with 20% for expansion: 1,031 × 1.2 = 1,238 l/min, or about 1.25 m³/min
The K 75 (1.1 m³/min) would be too tight. The K 110, 1.7 m³/min at 8 bar, covers this need with reserve to spare. For two booths, the K 75 is enough; for a workshop that adds presses and blow-off, the K 150 delivers 2.2 m³/min.
An undersized compressor is also a cause of defects: when demand exceeds its flow, the pressure drops at the gun and the glaze is sprayed in large droplets. The dryer, meanwhile, works beyond its capacity and the air comes out less dry.
Why do runs appear mainly in the morning or in summer?
Two situations come up again and again in ceramics workshops.
In the morning, after a cold night. Water that has condensed overnight in undrained pipes is carried along with the first air of the day. Fit an automatic drain on the air receiver and at the low points of the network, and let the network flush through for a few minutes before the first piece.
In summer, in hot and humid weather. The air drawn in contains more water, and a compressor room that is too hot pushes the dryer to its limit. Ventilate the room and keep the compressor away from the kilns.
Good habits in the booth
- One function per regulator. A blow gun sharing the spray gun's regulator makes the pressure vary every time it is used.
- Hoses kept for glaze only. A hose that has been used in the maintenance workshop may contain grease.
- A visual check of the filters. A booth filter filling with water signals a dryer that is switched off or overloaded. Change the filter elements when the manufacturer says.
- Compressor maintenance. First service at 500 hours, then every 1,500 hours. A worn air/oil separator lets more oil through to the filtration.
To size clean air for your booths, start with the configurator by choosing the ceramics activity, or describe your defects and your installation to us in a quote request.
For your trade: Compressor for ceramics and glazing.