Shampoo cleans with foam and mechanical agitation and uses little water; extraction flushes soil out with solution and recovers it by vacuum.

A rotary machine with shampoo works the foam into the pile, and the mechanical action of the brush is doing much of the cleaning. Soil is suspended in foam that then dries and is vacuumed away.

Extraction injects solution and immediately recovers it, physically removing soil and chemistry in the recovered water. It removes more soil overall and is the only method that flushes the pile.

Drying time is the main practical difference. Shampooed carpet is usable much sooner, which is why hotels and venues use it between deep cleans.

The usual professional pattern is shampoo or encapsulation as interim maintenance, with periodic hot water extraction as the restorative clean that removes accumulated residue and deep soil.

Traditional formulations did, badly. Modern ones dry to a brittle residue that vacuums out, provided the carpet is actually vacuumed afterwards.

Old shampoos left a soft, tacky surfactant film that held dry soil. Traffic lanes reappeared quickly and the method acquired a poor reputation that persists.

Current chemistry uses polymers that dry hard and brittle, encapsulating the soil so that both come out with the vacuum. The residue does not stay tacky.

That depends entirely on post-cleaning vacuuming. Skipping it leaves encapsulated soil in the pile, which then looks exactly like the old residue problem.

Over-application also defeats it - more product than the pile can dry out leaves surplus polymer behind.

Where a carpet with a shampoo history is extracted, expect heavy foaming in the recovery tank from old residue, and use a defoamer.

A rotary or oscillating machine with a shampoo brush for cut pile, or a bonnet for light interim cleaning of low-pile commercial carpet.

A shampoo brush works foam down into the pile and provides the agitation the method depends on. Brush stiffness should suit the fibre - a stiff brush on wool or a soft cut pile causes fuzzing.

Bonnet cleaning uses an absorbent pad on a rotary machine to clean the upper pile only. It is fast and dries quickly, and it is genuinely interim - it does not clean the lower pile at all.

Oscillating and counter-rotating brush machines are gentler on pile than a single-disc rotary and are preferred on delicate or long pile.

Machine weight and speed both affect the risk of distortion. Keep the machine moving; a rotary allowed to dwell in one place will mark the carpet within seconds.

Long or shaggy pile, delicate wool and hand-made rugs, and anything where the backing or dye cannot tolerate moisture and agitation.

Long pile tangles under a rotary brush and does not recover, so the appearance change is permanent.

Wool can felt under the combination of moisture, heat, alkalinity and agitation. Wool-safe chemistry at correct pH and a gentle machine reduce but do not remove the risk, and many wool carpet manufacturers specify extraction instead.

Hand-made and antique rugs should not be shampooed with a rotary at all - dye bleed and fibre damage are likely and irreversible.

Carpet with jute backing risks browning if over-wetted, though shampooing uses less water than extraction.

Check the carpet manufacturer's recommended cleaning method. Using a prohibited method can void a warranty as well as damage the carpet.

Usually a small number of hours, considerably faster than extraction - which is the main reason the method survives.

Because relatively little water is introduced, shampooed carpet returns to service far sooner. In hospitality and 24-hour operations that is often the deciding factor.

Air movement shortens it further. Air movers directed across the surface rather than down onto it are the standard tool.

Humidity and temperature matter as much as the method. A cold, humid building dries slowly whatever technique is used, and dehumidification may be needed.

Do not allow traffic before the carpet is dry. Walking on damp carpet re-soils it immediately and can transfer dye or residue to floors elsewhere.

Vacuum only when fully dry - vacuuming damp carpet with encapsulated soil achieves nothing and clogs the machine.

As the interim clean between restorative extractions, targeted at traffic lanes and high-use areas rather than whole floors.

A sensible commercial programme has three tiers: daily vacuuming and spot treatment, periodic interim cleaning of the areas that show wear, and less frequent restorative extraction of everything.

Shampooing and encapsulation both sit in the interim tier. They restore appearance quickly and cheaply and keep soil from building to the point where restoration is difficult.

Targeting matters. Cleaning entrance areas and main circulation routes frequently, while cleaning perimeter and low-traffic areas rarely, produces a better overall appearance for the same budget.

Interim cleaning does not remove deep soil, so extraction still has to happen on a schedule. A programme of interim cleaning alone eventually produces a carpet that looks acceptable and is heavily soiled.

The minimum that produces working foam - over-application leaves residue, extends drying and wastes chemical.

Shampoo is a concentrate diluted for the machine's tank, and the dilution should follow the label rather than being judged by how much foam appears.

Excess foam does not clean better. It leaves more polymer than the pile can dry out cleanly, holds moisture, and takes longer to vacuum away.

Heavily soiled traffic lanes may need a pre-spray before shampooing rather than a stronger tank solution, which puts the chemistry where it is needed.

Flow rate through the machine should be adjusted so the carpet is damp rather than wet, with foam visible in the pile and no free liquid.

Where the carpet stays foamy or feels tacky when dry, the dilution was too strong - and the correction is an extraction rinse, not more vacuuming.