Get this: you buy your raw materials by **weight** but you sell your product by **volume**. So if you make paint, adhesives and inks (even shampoo or chillie sauce), you must keep converting weight to volume. How do we do this?

This is when you pick up the data sheet and look for the SG. SG is short for **Specific Gravity**, also known as the **relative** **density** of something.

Specific Gravity is the **ratio** of the density (weight per unit volume) of a substance relative to the density of water, which we know is 1. So if the SG of something is less than 1 it floats in water and if greater than one, it sinks.

Since SG is a ratio, it has no dimensions. However in practice it is used as a measurement of density and can be given the unit **grams per millilitre** (g/ml or g/cm3 or even kg/l if you prefer).

Minerals and fillers are generally much denser than water, so their SG’s are greater than one. For example:

Specific gravity of common pigments and fillers | |
---|---|

Titanium dioxide | 4.1 |

Calcium carbonate | 2.7 |

Talc | 2.75 |

Barium sulphate | 4.5 |

Kaolin | 2.6 |

Dolomite | 2.7 |

The SG of a mineral is usually a **constant** for that mineral. The SG will therefore be the same for different grades and particle sizes of the same mineral.

When adding these solid powders to a liquid, we need to know what the volume added is. A simple inversion of the SG solves this riddle, e.g:

Volume of 1 kg of CaCO3 = 1/SG or 0.37 litres

(Density =Mass/Volume, so Volume =Mass/Density)

Do this with each component of the mix to find out the total volume of the formulation – easy as pie!

These fillers are often added to something much less dense, like **plastic**, **rubber** or **soap** to reduce its cost. Since the filler is denser than what it is replacing, the more you add, the smaller the volume of the final product.

There are clever ways to compensate for this smaller volume, like making grooves, indents or holes in the item. Then it looks the same size but has a smaller volume than before.

It is always a trade-off finding how much filler to add to get the most savings but not losing too much volume. If you add too much filler, you can also lose other good properties such as strength and flexibility.

SG should not be confused with another parameter you often see on data sheets, the **bulk density**. Bulk density gives an indication of how much volume a mineral takes up when packed, including the air spaces between the particles. This is useful to work out the space it will take up in a hopper or in storage.

For example, if the bulk density of kaolin powder is 0.7g/cm3 then you know that one ton of this powder will take up 1.43m3 of space (using Volume = Mass/Density again).

What I find interesting (being a scuba diver) is that human fat has a specific gravity of 0.94 while muscle tissue has a specific gravity of 1.06. So fat floats but muscle sinks! How much you have of each makes you either a floater or a sinker.

Now I know why I need such a heavy weight belt. Like this lady, I am definitely more of a floater than a sinker – what about you?