Compressed Air for Ceramics and Pottery Production

Compressed air is a hidden power line that runs through the entire process of ceramics and pottery production. From raw material processing, forming, glazing to kiln control, it doesn't get discussed as frequently as electricity or natural gas, yet it plays a decisive role in quality consistency and process precision. Most industry articles stop at "ceramics factories need air compressors." This article goes deeper, from the perspective of air source quality and process matching, to lay this line out thoroughly.

Oil-free compressors still produce compressed air that contains oil. "Oil-free" only means the compression chamber does not use lubricating oil, and the compressor itself does not add oil to the compressed air. The air that the compressor's intake draws from the environment already contains oil hydrocarbon vapors. Ceramics factories are usually located in industrial parks, surrounded by logistics vehicles, boiler flue gas, and emissions from neighboring factories, and the concentration of hydrocarbons in the ambient air is not low. These gaseous oil hydrocarbons pass through the oil-free compressor into the downstream piping network completely unchanged. ISO 8573-1 Class 0 certification only certifies that the compressor itself does not add oil contamination; it has never promised absolutely oil-free output air. Many ceramics factories spent big money purchasing oil-free machines, thinking they could skip the downstream oil-removal filters, only to find that the clogging rate of their inkjet line printheads did not decrease. This cognitive gap is quite common across the industry, including among a fair number of compressor sales engineers who haven't sorted this out themselves either.

The core raw materials of ceramics production, clay, feldspar, and quartz, are all micron-level powders. Once compressed air carries oil mist or condensate and comes into contact with these powders, it causes irreversible agglomeration, directly leading to uneven body density. This is not something the naked eye can detect during forming. It usually shows up only after firing, in the form of deformation, blistering, or color variation, by which point the loss is already irrecoverable.

Unlike metalworking or automotive spray painting, the sensitivity of ceramics processes to compressed air lies not in the absolute value of pressure, but in dew point stability, the sustained compliance rate of oil content, and the frequency characteristics of pressure fluctuations. A compressor's nameplate parameters being up to spec does not mean it will still be up to spec after three continuous days of operation, and ceramics kiln firing cycles are often in that range. Screw compressor lubricating oil degrades over operating time, and the volatile organic compound content of aged oil rises. Even with filters intact, the gaseous oil hydrocarbon content in the exhaust will gradually climb. The dust concentration in ceramics production environments is high and ambient temperature swings are large, so the oil deteriorates faster than under standard operating conditions. Shortening the oil change interval to about sixty to seventy percent of the recommended value costs very little but produces immediately visible results.

Among all processes, glazing and decoration is where compressed air has the most direct effect on product appearance, and where problems are most easily traceable back to the air source. So it goes up front for emphasis.

Glaze slurry is a suspension system, with particle sizes typically ranging from a few microns to forty or fifty microns. Oil mist in compressed air destroys the suspension stability of the glaze slurry when mixed in, causing flocculation. Trace amounts of oil adhering to the body surface carbonize during firing, forming pinholes or crawling. These defects are especially severe in reduction atmosphere firing, because under reduction atmosphere, carbonized material cannot be fully oxidized and expelled.

Silicone Contamination

The harm from silicone-type oils is on a completely different level from mineral oil. This needs to be discussed separately. Synthetic lubricating oils used in screw compressors, silicone sealants used at pipe connections, and even the silicone mold-release agents found on certain brands of PTFE tape can all become silicone contamination sources. Silicone has extremely low surface tension and very strong spreading capability on glaze surfaces. An extremely small amount of silicone contamination can cause widespread crawling, with a characteristic "fish-eye" distribution pattern where the center point is tiny but the affected area is large.

Digital Inkjet Printing

Digital inkjet printing has become the mainstream decorating process for ceramic tiles. The ink droplet volumes of printheads are extremely small, and nozzle diameters are extremely fine. The ink path system inside the printhead demands compressed air cleanliness at the ISO 8573-1 Class 1 level. Filter differential pressure rises over time, and after reaching a certain critical point, filtration efficiency drops suddenly rather than declining gradually. A filter assembly without differential pressure monitoring is essentially gambling.

The compressed air used on an inkjet line serves more than one function. Besides driving the ink path, compressed air is also used for printhead cleaning maintenance (purge cycle), during which the compressed air directly contacts the nozzle faceplate. Some factories connected a line of ordinary industrial compressed air to the cleaning system, and every cleaning cycle actually introduced new contamination into the printhead. This amounts to creating new wear every time maintenance is performed.

The air source issues in the glazing stage at least get some discussion. The kiln stage gets almost none.

The combustion system of a roller kiln requires compressed air to drive proportional control valves that regulate natural gas flow. The temperature precision required in the firing zone must be controlled within a very narrow range (down to within a few degrees for high-performance ceramics). The proportional valve response must be fast and linear, and the actuating mechanisms are almost all pneumatic. Once compressed air pressure fluctuates beyond a certain magnitude, the valve opening will shift, and the temperature curve deviates from target.

Rapid Cooling Zone

The rapid cooling zone of the kiln needs to precisely control the cooling rate during the descent from peak firing temperature to around 573°C (the quartz inversion temperature). Many roller kilns inject compressed air directly into the rapid cooling zone for temperature reduction. Uneven cooling triggers thermal stress, causing products to develop delayed cracking during storage after exiting the kiln. What makes this type of quality incident most frustrating is that it occurs after final inspection has already been passed.

The injection method of compressed air affects cooling uniformity far more than the injection volume itself. Single-side injection versus double-side staggered injection at the same air volume can result in cross-section temperature differentials differing by more than double. The bore diameter, spacing, and angle of the injection tubes, once determined at installation, are almost impossible to adjust afterward. This requires joint design with the compressed air system's supply capacity during the kiln installation phase. Many factories do it the other way around: build the kiln first, then try to match the air source. By that point the rapid cooling zone injection tubes are already welded in place, and the adjustable range is very limited.

After ball milling, the slurry goes through a spray dryer to produce powder. Airflow atomizers rely entirely on compressed air to tear the slurry into droplets. Airflow atomization has a mechanism that needs to be understood: the relationship between droplet size and gas-to-liquid ratio (GLR) is not linear. A critical zone exists. Below this critical value, atomization performance becomes exceptionally sensitive to pressure fluctuations, and small pressure disturbances cause dramatic changes in particle size distribution. During equipment selection, the compressor's output capacity must have sufficient margin above the atomizer's demand. The primary cause of batch-to-batch variation in spray granulated powder is often insufficient air source margin rather than problems with the atomizer itself. This attribution direction is frequently reversed during troubleshooting.

There is no clever solution for this stage. It comes down to doing two things properly: installing automatic drain valves at pipe low points and ensuring those drain valves are actually working (automatic drain valve clogging and failure is very common and requires regular checking), and ensuring that the dew point of compressed air used for conveying is low enough.

Cold isostatic pressing (CIP) is a key process for producing high-performance ceramics. The depressurization rate during the pressure release stage is critical: if compressed air is released too quickly, residual gas inside the body cannot escape evenly, forming lamination. This defect hides inside the body, appears completely normal externally, and only cracks along the lamination plane during firing shrinkage.

The safe upper limit for depressurization rate changes along with the body's powder particle size distribution, packing density, and body geometry. Coarse particle size bodies have good permeability and can tolerate faster depressurization; fine particle, high-density bodies have very poor permeability and require significantly slower depressurization rates. Equipment manuals typically provide a middle value, because equipment manufacturers' parameters need to cover all operating conditions and can only take a conservative middle position. Users need to run their own experiments based on their specific powder to find the optimal depressurization curve. There is no shortcut for this process.

The glaze spraying in handmade ceramics requires precisely adjustable pressure, with small and stable air volume. Any pulsation will leave visible streaks on the glaze surface. This won't be expanded on further. For small pottery studios, a small quiet compressor plus a manual pressure regulator, with attention to draining condensate from the lines, is sufficient. No complex system design is involved.

Engineering ceramics like alumina and silicon carbide have extremely high machining hardness, and pneumatic spindles need to reach very high rotational speeds. Pneumatic spindle bearing life is often determined by air supply quality rather than bearing quality itself. Leak testing for sanitary ceramics has very high pressure precision requirements. General-purpose industrial pressure regulators cannot do this job; precision regulators are needed. These two stages have high requirements for compressed air quality, but the logic is the same as the preceding stages: clean, dry, stable. The difference is that precision machining and inspection stages typically have small air consumption. A suitable set of precision filters and a miniature drying module at the point of use solves the problem without system-level modifications.

The vast majority of ceramics factories know their monthly total electricity bill and kiln natural gas consumption but do not know the production cost per cubic meter of compressed air. The compressor station has no separate electrical energy metering, and there is no total supply air volume flow measurement. Without these two data points, calculating unit cost is impossible.

When the production department requests adding a new air consumption point, decision makers cannot evaluate the operating cost. When the equipment department proposes investing in a heat recovery system or replacing compressors with more efficient ones, the finance department cannot calculate the payback period. Leakage rates in industrial compressed air systems are generally on the high side, and in loosely managed factories even more so. Without metering, nobody knows how much electricity the leaks are wasting every month. Installing a set of electrical sub-meters for the compressor station and a total pipeline flow meter costs no more than ten thousand yuan or so.

Condensate in compressed air piping networks does not disappear on its own. Even with automatic drain valves installed, small amounts of moisture still accumulate at pipe low points, dead ends, and unused branch lines. Workshop temperatures in ceramics factories tend to run high (especially near kilns). High temperatures combined with standing water cause biofilm to gradually form on pipe inner walls, composed of bacterial and fungal colonies, adhering as a slimy gel layer on the pipe surface. Biofilm periodically detaches from the pipe wall and travels downstream as gel-like fragments in the airflow, large enough to clog spray gun air cap passages or the precision air paths of inkjet printheads. The fragments are soft organic gel, not hard particles that conventional filters easily intercept. They deform and squeeze through filter media pores, meaning filters with rated precision far finer than the fragment size can still fail.

This problem is more pronounced in ceramics factories in warm, humid regions. Eliminating all dead pipe segments and unused branches in the network, ensuring all pipe segments have adequate slope and drain points, thoroughly flushing the network before restarting after extended shutdowns, and using activated carbon filters at critical use points (activated carbon's adsorption capacity for organic gels exceeds that of fiber filters) can control the biofilm problem when combined. It should be acknowledged, however, that biofilm contamination in compressed air systems is not well studied overall. There is a lack of systematic statistical data on what proportion of faults it specifically causes in ceramics production. The discussion here is based more on observation and inference from piping network maintenance.

If a ceramics factory has conducted compressed air quality testing, the sampling point is usually at the compressor station outlet or dryer outlet. The data at this location will look good, of course, because it is closest to the purification equipment. As compressed air travels from the compressor station through hundreds of meters of piping to the point of use, quality deteriorates continuously: corrosion and deposits on pipe inner walls release solid particles, residual moisture re-condenses in temperature differential zones, and aged seals on pipe fittings and valves release organic compounds. The quality grade measured at the compressor station outlet, by the time it reaches the inkjet line terminal two hundred meters away, may well have dropped two grades or more.

The dew point of refrigerated dryers fluctuates with ambient temperature and load. Under summer high-temperature, high-humidity conditions, the pressure dew point of a refrigerated dryer can rise noticeably. For spray drying, powder conveying, and inkjet printing, this magnitude of rise is unacceptable.

A combined configuration of refrigerated and adsorption dryers is more reasonable. Adsorption dryers come in heatless regeneration and heated regeneration types. Heatless regeneration has a high purge air loss rate; heated regeneration has a much lower loss rate but higher equipment investment. With the large air consumption of ceramics production, heated regeneration is more economical over the long run.

Most adsorption dryers use fixed-time cycle switching. Inlet air humidity varies substantially with seasons and between day and night. During dry winters, fixed-cycle switching means frequent unnecessary regeneration. During humid summers, regeneration may be insufficient. Models equipped with dew point controlled switching logic can dynamically adjust regeneration cycles based on outlet dew point. Under the long continuous operating conditions of ceramics production, the savings are considerable. Exactly how much can be saved depends on local climate conditions and air usage patterns, and varies greatly between factories. No universally applicable number can be given.

Ceramics factories have large workshop spans and scattered air consumption points. Poorly designed piping networks are the primary source of pressure fluctuations. Ring main networks suit ceramics factories better than branch networks. Each critical use point should have a receiver tank for buffering.

Carbon steel pipes produce rust over long-term use. Rust particles entering the glaze line with compressed air form iron spots after firing. Aluminum alloy or stainless steel piping has higher initial investment but is a more reliable choice in the ceramics production environment. Horizontal pipe runs must maintain a downward slope in the direction of airflow, with automatic drain valves at low points. The plant structures of ceramics factories are complex, and pipe routing frequently needs to cross over beams and columns. If no drain point is installed after the crossover, condensate accumulates at reverse bends and is eventually carried by the airflow into downstream use points. This problem is invisible on installation drawings and can only be judged during on-site pipe routing, yet many installation crews lack this awareness. The same issue was mentioned in the powder conveying section above: automatic drain valve clogging and failure is very common. Drain valves that are installed but functionally equivalent to not being there are everywhere in ceramics factories.

Pressure Standing Waves

The piping network also exhibits another phenomenon: pressure standing waves. Screw compressor exhaust contains small pulsations related to rotor speed. When these pulsations propagate through long pipes and reflect off closed end valves at terminals, the incident and reflected waves superimpose to form standing waves that produce stable pressure low points at specific locations in the piping. If a use point happens to be located at a standing wave node, the supply pressure at that location will be persistently low even when the network's average pressure is perfectly normal. Ordinary pressure gauges do not have sufficient sampling frequency to capture this high-frequency fluctuation. How prevalent this situation is, is hard to say, because most factories have never performed frequency spectrum analysis and don't even know whether the problem exists. In piping networks with longer total length and multiple dead-end branches, the probability of occurrence is relatively high. Solutions include installing silencer buffer tanks in the network to attenuate pulsation energy, or adjusting the connection position of use points to the network. This topic has a degree of theoretical character. For most ceramics factories, if a particular use point has persistently low pressure and no leak can be found, it can serve as an investigation direction. Not every factory needs to run a full spectrum analysis.

Air consumption in ceramics production fluctuates widely: low during pre-ignition kiln preparation, highest during full-load production, with sudden high-volume surges when the rapid cooling zone activates. A single fixed-speed compressor adjusts output through load/unload cycling, still consuming a significant amount of rated power during unloaded operation.

A multi-unit coordination scheme with a variable frequency main unit plus fixed-speed auxiliary units is more reasonable. The variable frequency unit handles the fluctuating portion. Fixed-speed units handle the base load. A central controller dispatches automatically based on network pressure signals.

Screw compressors convert the majority of input electrical energy into heat during operation. Body drying in ceramics factories requires large amounts of low-temperature heat, and compressor waste heat falls right in that temperature range. By adding a waste heat recovery heat exchanger, the heat from compressor cooling water can be routed into the drying kiln's hot air system, while simultaneously reducing cooling water temperature and improving the aftercooling of compressed air. Lubricating oil operating temperature also drops as a result, slowing oxidation and allowing the oil change interval to be extended correspondingly. The drying kiln saves thermal energy, aftercooling performance improves, and lubricating oil life is extended. The payback period for this modification is usually not long, though exactly how long depends on local electricity and natural gas prices, the drying kiln's thermal load, and the compressor's operating conditions. A specific heat balance calculation is needed.

ISO 8573-1 classifies compressed air quality across three independently graded dimensions: solid particles, moisture content, and oil content. Different process stages in ceramics production correspond to very different quality grades. Powder conveying and general pneumatic tools have modest requirements. Glazing needs a medium level. Inkjet printing and precision testing need the highest level. Configuring the highest grade air source for all use points within a single factory is not only economically unreasonable, it is also technically incorrect. As already mentioned, excessively high levels of drying treatment in the powder conveying stage actually worsen static problems. Tiered air supply is the right approach: configure basic-level purification equipment on the main pipe, and add end-point precision filters and drying modules before high-requirement use points.

Many quality issues in ceramics production get attributed to raw material fluctuations or kiln tuning problems. The root cause may be in the compressed air.

Regular pitting on body surfaces with normal density test results could be caused by condensate intermittently entering powder from dense phase conveying pipes. Localized clusters of pinholes on glaze surfaces after spraying could be caused by organic material leaching from aged sealing material at a pipe joint somewhere in the compressed air line. Unexplained fluctuation in the kiln firing curve at a certain temperature stage could be caused by unstable air source pressure at the proportional valve. These are all "could be," because the causes of these defects are multifactorial, and compressed air is only one suspect. Including air source issues in the investigation scope, rather than fixating exclusively on raw materials and kilns from the start, can avoid a lot of dead ends.

Regular compressed air quality testing should be incorporated into the quality management system. A more ideal approach is to install online monitoring instruments at critical use points connected to the data acquisition system, performing correlation analysis between compressed air quality data and product quality data. Very few ceramics factories currently achieve this. Most factories' SPC systems monitor raw materials, forming parameters, and kiln temperature curves, but not compressed air.

Everything that needed to be said has more or less been said. Compressed air has low visibility in ceramics production, yet when problems arise, it is often the last investigation direction anyone thinks of. The compressor station people do not understand ceramics processes. The kiln and glaze line people do not understand pneumatic systems. Each side manages its own area, and problems at the interface go unaddressed. Until this situation changes, the same quality issues will keep recurring.