How clean should a carpet be after it has been freshly cleaned? No other question will perplex most carpet cleaners as this very question, and with good reason.
The dictionary definition of clean is free from dirt, marks, or stains. So should a carpet be free from dirt, marks or stains once it is cleaned? Hardly! Hardly you said? Indeed. Let me explain.
No other question in the world of cleaning is so controversial as to define what is an acceptable level of clean, but we must recognise that complete of exhaustion of soil from a carpet is limited by sheers physics and time contraints. We can only ever reduce the level of soiling in a carpet.
Let me give some real-world examples to illustrate this before a semi-technical explanation.
Example 1. Take your dirty car to a DIY pressure cleaning car wash, and give it a thorough blast and clean. Let it dry in the sun. Then re-wet a section of the car with some water some water and wipe it with a white cloth. You will notice that there is some level of soil transfer to the cloth.
Example 2. Throw a freshly cleaned load of laundry back through the washing machine, and collect the effluent from the second wash in a bucket. I bet its still got some dirt in it!
Unfortunately, we can not get even close to the level of clean that either of the examples above are able to achieve on a fixed, wall to wall installation of carpet, unfortunately. Here is one semi-technical explanation as to why.
For the illustration, I will let you imagine a cup full of urine. Yuk! If I empty the cup and rinse it once with water, would you be prepared to drink from it? Of course not! You instantly recognize that a small amount of urine has clung to the inside wall of the glass, and some small quantity of urine is still going to be present after one rinse.
When we clean a carpet with an extraction wand, the same limitations are at play. The individual carpet fibre is under vacuum, and the soil embedded emulsion closest to the fibre moves at the least velocity due to parasitic drag. This is not intended to be a discussion on fluid dynamics, but in fact the physics governing fluid motion guarantees that some level of soil will always remain. With the carpet cleaning process, we are essentially undertaking a process of DILUTION, NOT complete EXHAUSTION/REMOVAL.
The quantity of soiling that is removed from a carpet, and the quantity of soil that remains is difficult and impractical to measure. The best practical tool we have to getting acceptable soil reduction levels is utilising the steam cleaning (hwe) process cleaning system recognised in the standard AS/NZS 3733 Textile Floor Coverings – Cleaning Maintenance, which is the most effective soil reduction process, through to the lesser capable dry cleaning (low moisture) systems, coupled with experience (to know which system is of best fit). There exists no acceptable and recognized method for concluding whether a carpet has been cleaned properly other than to review the regime undertaken by the carpet cleaner, and compare it to the standard. Re-wetting a carpet and wiping it with a white cloth will yield some soil transfer on a freshly cleaned carpet for all the reasons specified above, and is NOT an acceptable measure of the quality of a carpet cleaning service. In fact, AS/NZS3733 tacitly acknowledges that soil will be removed from a freshly cleaned carpet, by stating the carpet cleaning process needs to be followed up with post vacuuming upon drying of carpets AFTER cleaning. The purpose being to remove unrecovered dry soil.
The take away message in all of this is that regular carpet maintenance, which includes bi-weekly vacuuming (preferably with an upright, cylindrical brush vacuum or equivalent), and periodic professional carpet cleaning (cleaning before large quantities of soil are visible), is the best way to promote hygiene, appearance and longevity of a carpet, and to keep soil load to a minimum.