Book Extract #6: Why is Dirt Dirty?

Perhaps the most common type of stain is common garden variety (pun intended) dirt.

Exactly what is dirt?  Consider the following scenario, which many of you may be familiar with.  You have just built a new house and you want to put some turf down to create a lawn.  You get in some yellow builders sand, lay it down, level it, then roll your turf on top.

Several months later, after some fertiliser and watering, the turf has bedded in and you have a nice luxuriant lawn.  You decide to add a water feature so you dig up part of the lawn, only to discover that the nice yellow sand has disappeared and its now black, down as far as your shovel has penetrated. In other words, the sand has been replaced by dirt.

What happened?  What’s the black stuff?

Essentially, the black stuff is decayed plant matter called humic and fulvic acids.  Unlike other acids these do not have a well-defined structure, but are large, complex organic structures that contain many functional groups.

Other things in the dirt are mostly inorganic – sand itself is composed of boron and silicon oxides (anything with the word “oxide” in it is inorganic).  On top of this there are calcium and magnesium salts and of course iron oxides (this is the reddish stuff in soils).

A major component of dirt is of course clay. Although mostly composed of aluminium silicates, clay also contains iron, magnesium, sodium, potassium, phosphorus and other inorganic species present mostly as oxides.

Dirt, therefore, can essentially be considered to be inorganic in nature.  Its attachment to clothes is mostly physical.  That is, it may be considered essentially as big lumps of stuff wedged between the fibres.  An important consideration is that dirt contains many oxides.  Oxides in general are very chemically stable, and this essentially makes them resistant to chemical attack.  That is, unlike most stains, you cannot dissolve dirt by changing its chemical form – you need to remove it physically (oxides can of course be dissolved chemically, but the chemicals required to do it would also dissolve your clothes!)

Another major form of inorganic stain is rust, from being in too close proximity to a screw or nail.  This is in a class of its own in terms of stain removal, and it is discussed elsewhere.

9590cookie-checkBook Extract #6: Why is Dirt Dirty?