Operators also check clothing for foreign objects before placing them into the machine. Items such as plastic pens will dissolve in the solvent bath and may damage textiles beyond recovery. Some textile dyes are "loose" (red being the main problem), and will release dye during solvent immersion. Operators will dry clean red clothing separately from lighter-color textiles to avoid color transfer. The solvent used must be distilled to remove impurities that may transfer to clothing. Operators checked clothing for dry-cleaning compatibility, including fasteners. Many fasteners that decorate clothing either are not dry cleaning solvent proof or will not withstand the mechanical action of cleaning. Operators will remove them and restitched them after the cleaning. Or, operators will wrap them in a small padded protector. Fragile items, such as feather bedspreads or tasseled rugs or hangings, may be enclosed in a loose mesh bag. The density of perchloroethylene is around 1.7 g/cm³ at room temperature (70% heavier than water), and the sheer weight of absorbed solvent may cause the textile to fail under normal force during the extraction cycle unless the mesh bag provides mechanical support.
Many people believe that all marks or stains can be removed by dry cleaning. Not every stain can be cleaned just by dry cleaning. Some need to be treated with spotting solvents; sometimes by steam jet or by soaking in special stain remover liquids before garments are washed or dry cleaned. Also, garments stored in soiled condition for a long time are difficult to bring back to their original color and texture. Natural fibers such as wool, cotton, and silk of lighter colors should not be left in dirty or soiled condition for long amounts of time as they absorb dirt in their texture and are unlikely to be restored to their original color and finish.
The main reason that washing in water has a different result than dry cleaning is that water is a polar solvent while dry cleaning uses a non-polar solvent.
A typical wash cycle lasts for 8–15 minutes depending on the type of garments and degree of soiling. During the first three minutes, solvent-soluble soils dissolve into the perchloroethylene and loose, insoluble soil comes off. It takes approximately ten to twelve minutes after the loose soil has come off to remove the ground-in insoluble soil from garments. Machines using hydrocarbon solvents require a wash cycle of at least 25 minutes because of the much slower rate of solvation of solvent-soluble soils. A dry-cleaning surfactant "soap" may also be added.
At the end of the wash cycle, the machine starts a rinse cycle wherein the garment load is rinsed with fresh distilled solvent from the pure solvent tank. This pure solvent rinse prevents discoloration caused by soil particles being absorbed back onto the garment surface from the "dirty" working solvent.
After the rinse cycle, the machine begins the extraction process, which recovers dry-cleaning solvent for reuse. Modern machines recover approximately 99.99% of the solvent that was used. The extraction cycle begins by draining the solvent from the washing chamber and accelerating the basket to 350 to 450 rpm, causing much of the solvent to spin free of the fabric. Until this time the cleaning is done in normal temperature, the solvent is never heated in dry cleaning process. When no more solvent can be spun out, the machine starts the drying cycle.
During the drying cycle, the garments are tumbled in a stream of warm air (60-63°C/140-145°F) that circulates through the basket, evaporating any traces of solvent left after the spin cycle. The air temperature is controlled to prevent heat damage to the garments. The exhausted warm air from the machine then passes through a chiller unit where solvent vapors are condensed and returned to the distilled solvent tank. Modern dry cleaning machines use a closed-loop system in which the chilled air is reheated and recirculated. This results in high solvent recovery rates and reduced air pollution. In the early days of dry cleaning, large amounts of perchlorethylene were vented to the atmosphere because it was regarded as cheap and believed to be harmless.
After the drying cycle is complete, a deodorizing (aeration) cycle cools the garments and removes even more of the solvent, by circulating cool outside air over the garments and then through a vapor recovery filter made from activated carbon and polymer resins. After the aeration cycle, the garments are clean and ready for pressing/finishing.
In fact, dry cleaning does not remove all of the solvent. There are no government standards for how much solvent is allowed to remain in the fabric. Detectable amounts of the solvent remain in clothes, with wool clothes retaining the most solvent.
Working solvent from the washing chamber passes through several filtration steps before it is returned to the washing chamber. The first step is a button trap, which prevents small objects such as lint, fasteners, buttons, and coins from entering the solvent pump.
Over time, a thin layer of filter cake (called muck) accumulates on the lint filter. The muck is removed regularly (commonly once per day) and then processed to recover solvent trapped in the muck. Many machines use "spin disc filters," which remove the muck from the filter by centrifugal force while it is back washed with solvent.
After the lint filter, the solvent passes through an absorptive cartridge filter. This filter is made from activated clays and charcoal and removes fine insoluble soil and non-volatile residues, along with dyes from the solvent. Finally, the solvent passes through a polishing filter, which removes any soil not previously removed. The clean solvent is then returned to the working solvent tank.
To enhance cleaning power, small amounts of detergent (0.5%-1.5%) are added to the working solvent and are essential to its functionality. These detergents help dissolve hydrophilic soils and keep soil from redepositing on garments. Depending on the machine's design, either an anionic or a cationic detergent is used.
Since the solvent recovery is less than 100%, and because dry-cleaning does not remove water-based stains well, entrepreneurs have developed the wet cleaning process, which is, in essence, cold-water washing and air drying, using a computer-controlled washer and dryer. In general, wet cleaning is still being improved and developed, although low-tech versions of it have been used for centuries.
People who make clothes have special symbols to say how they should be cleaned. The international GINETEX laundry symbol for dry cleaning is a circle. It may have a letter P inside to indicate perchloroethylene solvent, or a letter F inside to indicate a hydrocarbon solvent. A bar underneath the circle means that only mild cleaning processes should be used. A crossed-out empty circle means the clothes should not be dry cleaned.
Cooked Powder Residue — the waste material generated by cooking down or distilling muck. Cooked powder residue is a hazardous waste and will contain solvent, powdered filter material (diatomite), carbon, non-volatile residues, lint, dyes, grease, soils, and water. Laws say how this material can be discarded.
The waste sludge or solid residue from the still contains solvent, water, soils, carbon, and other non-volatile residues. Material removed from the bottom of the stills at chlorinated solvent dry cleaning operations are hazardous wastes.