Tephrabase: Acid digestion of organic samples for the extraction of tephra

The aim of this is to obtain a clean mineral residue sample, free from organic material which is suitable for scanning under a petrological microscope or for electron microprobe analysis.  Sediments are treated chemically to remove organic matter, but to leave silica (biogenic and otherwise) unaffected.  Tephra and a variety of silica microfossils may be present in the treated sample. 

The technique described below is a modified version of Christer Persson’s (1971) method and and does not affect the geochemistry of the glass shards (Dugmore et al., 1992).  It is suitable for organic rich samples of a few cm3.

It is important that contamination of samples be avoided.  Always use clean glassware, stirrers, pipette tips etc.  Always clean glassware and equipment thoroughly and return to the correct storage area after use.


This technique involves using hot concentrated acids. It is, therefore, potentially dangerous. Make sure that you follow your institution's health and safety guidelines when carrying this out. It is very important that both you and your laboratory are equipped and competant to undertake this work. We cannot accept any responsibility for any accidents as a result of following these instructions.

Also please note, that these instructions are specific to the laboratories in Geography, University of Edinburgh and may not be applicable in other places.


  1. Place between 2 to 5 cm3 of sediment in a 250 ml Pyrex (screw-top) conical flask.  Keep the screw-top in a safe place and return it on the flask to the shelves when finished.  Clearly label each flask a number of times - during an acid digestion pen marks can be erased; the more marks the better the chance that the flask will remain labelled during the procedure.  Also always place your flasks in order, this reduces the risk of confusing which flask is which if the pen is erased.
  2. Place a large hotplate in the centre of  a fume cupboard, at this stage do not switch the hotplate on.  DO NOT PLACE THE HOTPLATE CLOSE TO THE PLASTIC SIDES OF THE FUME CUPBOARD.  Ensure that the fume cupboard extractor fan is ON.


  1. Place the conical flasks containing the samples in the fume cupboard. For samples of approximately 1 to 2 cm3 slowly add approximately 50 ml of concentrated Sulphuric acid 98% SLR (Standard Laboratory Reagent).  For larger samples (2 to 5 cm3) increase the amount of Sulphuric acid to a maximum of 100 ml.  If the samples are wet they may react and spit a little with the Sulphuric acid.
  2. Gently swirl the conical flasks and place them on the hotplate. Switch the hotplate on and set the thermostat to 10 (ten).


  1. Gradually the contents of the flasks will begin to simmer and white fumes will be driven off.  Very organic samples may produce a bubbly froth which fails to disperse and continues to rise in the flask. If the contents of the flask rise above the 250 ml mark then remove the flask from the hotplate and leave to cool within the fume cupboard and eventually the froth will subside.  The flask can then be returned to the hotplate. CAUTION, the outside of the flask(s) will be hot, even with gloves on.


In the event of a spillage or if the foaming contents of a flask has been left to boil over the hotplate and the fume cupboard work surface. DO NOT PANIC as this usually only results in the partial loss of one sample and if kept contained within the fume cupboard there is limited danger to the person involved.  First, remove the flask from the hotplate and place to one side.  Then, rinse and dry your gloved hands under a tap and switch off the hotplate.  Then, using the emergency trigger operated spray wash, gently hose the fume cupboard work surface directing the spray downwards and washing the acid spillage towards the rear drain trough.  Do not spray the hotplate or the flasks still on it.  Any acid on the hotplate can be left to evaporate away. Carefully wipe the sides of the spilled flask with wet tissue and dispose of acid soaked tissue into the Acid Waste bin.  If sufficient sample remains in the flask (the acid content can be topped up) it can be returned to the hotplate and the hotplate switched back on.

  1. The contents of the flask will eventually reduce to a stable simmering black liquid and will show no signs of frothing.  At this stage the samples may be left unattended (Complete the ‘Equipment Running Unattended Notices’ on the side of the fume cupboards).  If leaving the flasks unattended please lower the fume cupboard sash.
  2. After 1 to 2 hours of simmering using a plastic 3 ml pasteur pipette add approximately 5 drops of Nitric acid S.G. 1.42  68-72% SLR.  The contents of the flask may violently spit when the Nitric acid is added. Therefore, it is important to be wearing a white lab coat, PVC apron and eye protection to avoid small acid burns on skin or clothing. The liquid contents of the flask will produce brown fumes and a small amount of foaming that should be under control.  Leave the flasks to simmer on the hotplate for another 30 minutes to 1 hour. 
  3. Add some more Nitric acid (as in step 7.) increasing the number of drops to approximately 10 (ten). Then leave to simmer for another 30 minutes to 1 hour. If the flasks show signs of boiling dry then it may be necessary to top up the flasks with some more Sulphuric acid.
  4. After repeating step 8 eventually the liquid contents of the flasks will turn a clear yellow liquid, sometimes to almost a clear colourless liquid.  Once the liquid has turned clear the flasks may be removed from the hotplate and left to cool.  Switch the hotplate off when the last flasks has been removed.
  5. Once cool VERY SLOWLY add distilled water.  Initially the liquid in the flasks will froth violently and brown fumes will be driven off but as more distilled water is added slowly the contents settle and the flasks can be filled to the top.  Leave the flasks in the fume cupboard to allow the sediment content to settle.  The fume cupboard extractor fan should remain switched ON as acid vapours will still be evolved from the flasks.
  6. Once the sediment has settled using a clean glass pasteur pipette, attached via a length of tubing to a water vacuum pump, draw off the liquid content of the flasks leaving approximately 30 to 40 ml of liquid at the bottom and being very careful not to disturb the sediment at the bottom. Clean the pasteur pipette by drawing through distilled water between each sample.
  7. Transfer the remaining liquid and sediment content of the flasks to 50 ml Falcon conical centrifuge tubes (correctly labelled). Before putting in centrifuge ensure that the orange caps are securely fitted to the centrifuge tubes.  Then centrifuge the tubes at 3,000 rpm for 5 minutes. ALWAYS ensure that the centrifuge tubes have the same amount of fluid in each to be balanced.


  1. Once it has cooled, remove the hotplate and any other laboratory materials from the fume cupboard so that it is empty.  Lower the sash and operate the wash down facility. 
  2. Carefully decant the liquid from each centrifuge tube leaving the sediment at the bottom of the tube.  Then add 50 ml of distilled water to each tube ensuring that the sediment is well mixed.  Then centrifuge the tubes at 3,000 rpm for 5 minutes (as step 12).
  3. Repeat step 13 until all the residual acid has been washed from the mineral residue.  This may take 4 to 5 repeat washes with distilled water.
  4. After the final centrifuging, when the acidity of the sediment has been neutralised (check with pH paper, decant the liquid and then carefully transfer the sediment into an appropriately sized specimen tube and ensure that the tube has been correctly labelled giving details of site, depth, your name and date.
  5. Clean up the area of the Laboratory where you have been working. The 250 ml conical flask(s) can be washed using the Lancer dishwasher (please follow the dishwasher instructions for loading of glassware and operation of the machine). Once dry all the glassware should be returned to the correct storage area.

© Anthony Newton, Bob McCulloch and Andrew Dugmore (1992-2005)