Analytical Methods:Lab PSD

SASSA Home Page &#8658; Analytical Methods Home Page &#8658; Laboratory Techniques &#8658; Laboratory Particle Size Analysis

Particle Size Analysis
Particle size distribution is a measure of the % mass or % volume of particles in particular size classes. Essentially this is a more accurate measure of soil texture.

Questions
Particle size analysis is used primarily in studies of depositional conditions. For example was this deposit water-lain or wind-lain, and was this deposit deposited from moving water or stagnant water. Particle size analysis may also be used to identify sources of materials in anthropogenic deposits and to identify whether there is a mixture of materials which may result in a bimodal particle size distribution.

Example case studies where particle size analysis has successfully been applied to archaeological questions include:


 * Alluvial
 * Bimodal archaeological deposit
 * Clay illuviation

Sampling
Bagged bulk samples are all that is usually required for particle size analysis. Samples should be taken from a representative part of a single deposit. The sample should not bulk together more than one deposit. Replicate samples are needed if there is reason to think that the deposit will contain a range of grain sizes that it would be wrong to bulk together. For example, many water-lain deposits may fine either vertically or laterally. Multiple samples from a single deposit may be necessary to identify and measure this fining. Inorganic particle size distribution is relatively stable and hence samples can be safely stored. Samples will keep better if they are air-dried first.

Analysis
Air-dried soils are sieved to 2 mm to remove any stones; if this is a coarse deposit these may be weighed and recorded separately.

For further analysis the soil may need to be treated to remove any organic matter or calcareous material that can cause individual clay particles to clump together (flocculate). As a rule of thumb, if your soil contains more than 5% organic matter (analysed using loss-on-ignition) or has a conductivity of more than 2 dS/m, it will need pretreating.

More information on pretreating can be found here.

Any analysis of the silt and clay fraction also requires the addition of calgon to prevent clumping.

There are many different methods for particle size analysis but these split into three main groups. It uses the principle that coarse particles separate out of a suspension quicker than fine particles. The clay and silt content is measured by either drawing off samples by pipette (pipette method) or using a hydrometer to measure suspension density (hydrometer method) over set periods of time.
 * Sieving – Physical separation of grain sizes through a stack of sieves of reducing mesh size down to a minimum of 60 μm. Whilst smaller mesh sizes can be used to quantify the silt fraction down to 5 μm the time investment required means that one of the other techniques should be used for the fine fraction.
 * Sedimentation – This procedure is usually carried out on the less than 60 μm fraction (silt and clay) after the sand has been quantified by sieving.

More information on the pipette method can be found here.


 * Laser diffraction analysis – Uses specialist equipment to calculate particle size distribution based on the scattering of light passed through the suspended sample. The advantage of this technique is that it produces a breakdown of the complete spectrum of grain sizes and hence can be more sensitive to changes in particle size distribution.

All these techniques make assumptions about grain shape, usually assuming grains are spherical. Laser diffraction also makes assumptions about the optical properties of the grains.

Data and Interpretation
The data is usually presented in the form of size class percentages of the total weight (mass) of the sample. Laser diffraction data may also be presented as % of the sample volume. Sizes are usually given as millimetres (mm) and microns (μm), or phi (φ). Results may also be graphed as a percentage curve or cumulative percentage curve.

More information on particle size classes can be found here.

Related techniques
Organic matter content

 &larr; Back to Laboratory techniques