The other day a client asked me what the difference was between kaolin and talc. Hmm, good question. Well before getting to the differences, let’s first see what they have in common.
They are both fine white powders that come from digging a mineral out of the ground and refining it. If you take a microscope and look at the particles close-up, they both look like tiny flat plates, which are called (surprise, surprise), platelets. This platelet shape makes them good at cutting out the light, which is why we put them into paints and ceramic glazes to help with opacity.
Kaolin and talc have a lot more in common: They are both inert, which means they take no part in chemical reactions. They are also both insoluble in water, so they do not dissolve (although it looks like they do when all the particles are dispersed and form a milky solution).
Although both are minerals, only kaolin is called a clay. In fact kaolin’s common name is china clay. The common name for talc is talcum powder and it is the softest mineral on earth. That must be why it is chosen for use on babies bottoms!
Kaolin is also a soft mineral and comes just above talc on the Mohs hardness scale of minerals:
| Mohs hardness of minerals | |
|---|---|
| Diamond | 10 |
| Silica | 7 |
| Kaolin | 2 |
| Talc | 1 |
Looking at a pile of kaolin powder and a pile of talc powder, however, you will notice an obvious difference. The talc is much whiter (unless it is very impure). If you put your hand into the powder, you will find that although the kaolin makes your hand white and dusty, the talc sticks to it like blazes. And to your clothes, your face, the dog, etc.
That’s the biggest difference, I think. Talc is hydrophobic which means water-hating. Hence it loves anything organic like you, your face, the dog…..you get my drift.
Kaolin, on the other hand, loves water so it is hydrophilic. That makes it much easier to disperse into water than talc, which tends to sit on top of it. Talc may be water-hating but surprisingly, you can still disperse it into your water-based paint without too much of a problem. This water-hating property does come in useful in waterproofing products, where you need ingredients that help to reject water.
Open a chemistry book (does anyone still do that these days?), OK strike that, go to Wikipedia, and it will tell you that kaolin is a clay mineral and falls into a family called alumino-silicates. Talc, however, is a magnesium silicate. This would explain their different behaviour. However sometimes when you are just using them as inert fillers, their differences are not important and you would just buy whichever is cheapest – usually kaolin. Like when you are using them in laundry soap bars, for example. For white toilet soap, though, you will need whiteness, so then you would choose talc rather than kaolin.
It gets even more interesting when you melt these two minerals, as happens when they are used in ceramics. Kaolin is a basic ingredient of most ceramic bodies and glazes and is needed for its alumina content, while talc is used mainly as an additive which helps to control thermal expansion, reduce crazing and improve the whiteness of the ceramic article or glaze.
Just like a brother and sister, these two minerals sometimes work well together and sometimes they compete with each other. But we love them both, just the same (OK, I admit it, I prefer kaolin, but that is only because it has been my baby for so long…)