Euparal Mounting Medium

Euparal is slightly yellowish in color, flows well and cures after a few days. After curing, it becomes very hard but not brittle, keeping elasticity. Euparal also adheres strongly to glass. It has a refractive index of 1.5174.

Compared to other mounting media, Euparal has a significant advantage: Specimens can be directly transferred from alcohol into Euparal. Other mounting media, such as Canada Balsam, require the specimen to be transferred to xylene (toxic) prior embedding.

Euparal was first described by G. Gilson (prof. of Zoology at Louvain University, Louvain, Belgium) in 1906, and is liked for its ease of use and stability.

I wanted to find out more about this mounting medium and conducted a quick Web search, only to find out that the information is scarce. I wanted to know more about the composition of Euparal, also because of safety considerations. Many non-aqueous mounting media contain toxic organic solvents, which I try to avoid.

After some time I was indeed able to find the original publication by G. Gilson [1], published in French. An article published in the Journal of the Royal Microscopical Society [2] summarized parts of this his and together with a translation software, I was able to extract some meaning of Gilson’s article.

Composition of Euparal

Gilson found out that Sandarac (or sandarach) is a suitable resin for mounting. It is obtained from Callitris quadrivalvis, a tree belonging to the cedar family. The resin has been previously used to make varnish and protective coatings for paintings. Alcohol could disolve the sandarac well, but the resulting medium was not suitavle for microscopic work. During the curing process the sandarac started to crystalize and crack.

Gilson then mixed the sandarac with camsal, a mixture of phenyl salicylate (salol) and camphor. The camsal was not a good solvent for the sandarac, but prevented the formations of crystals and cracks. He added either isobutylic or propylic alcohol to further dissolve the sandarac. Especially isobutylic alcohol was considered suitable, as it was commonly used to dehydrate microscopic specimens. The specimens could then be directly transferred into the mounting medium (containing the same solvent).

While this was aldready a step into the right direction, the added alcohol was a substantial disadvantage. Stained specimens could not be mounted with this medium, as the alcohol dissolved many the dyes which are commonly used in microscopy, such as eosin, safranin, methyl green were affected by the alcohol. Camsal alone was not able to sufficiently dissolve the resin.

Gilson, therefore, searched for replacements for the alcohol. He discovered that a combination of eucalyptol and paraldehyde was able to substitute for the alcohols, without harming pigmentation. He thus gave the mixture containing sandarc, salol, EUcalyptol and PARAldehyde the name Euparal.

  • Sandarac: A resin which solidifies in air. Originally used as a varnish for furniture.
  • Paraldehyde: Preservative and solvent.
  • Eucalyptol: Solvent of Euparal. dominant portion of Eucalyptus globulus oil. eucalyptol is used as an insecticide.
  • Phenyl salicylate (salol): Antiseptic substance, was introduced in 1886 under the name Salol, a desinfectant.
  • Camphor: An antimicrobial substance, previously used for embalming. Obtained from the evergreen tree camphor laurel (Cinnamomum camphora).
  • Camsal: A mixture (1:1) of camphor and Phenyl salicylate (salol).

Advantages of Euparal

Towards the end of the article, Gilson lists several advantages of Euparal. These advantages are now briefly summarized:

  • It is possible to directly transfer specimens which were stored in 70% alcohol into Euparal for permanent mounting. It is not necessary to completely dehydrate the object by placing it into absolute alcohol.
  • It has a low refractive index (1.481), which can be an advantage to observe certain structures. Other publications consider this low refractive index a disadvantage, however.
  • Euparal can be colored green (“Euparal vert”) by adding some copper salt. This can further increase the contrast of specimens stained with hematoxylin.
  • Euparal possesses reducing properties and therefore prevents oxidation of some dyes (such as hematoxylin).
  • Last, Euparal possesses good fluidity, does not pull strings and handles easily.

I now would like to mention a few advantages of Euparal:

  • Unlike other non-water-based mounting media, Euparal does not use the harmful solvent xylene
  • Euparal cures relativley quickly
  • And on the less serious side: Euparal smells nicely (but don’t inhale, nevertheless!!! Irritant and flammable!).

Euparal does possess some disadvnatages as well:

  • Acid sensitive dyes do not keep well, when embedded in Euparal.
  • Euparal does have the tendency to shrink a bit. This can introduce air bubbles.
  • It is flammable and an irritant. Eye and skin contact must be avoided.
    • References

      [1] Gilson, G. (1906). La Cellule, Vol. 23, pp. 425-432.
      [2] Hebb, RG., ed., (1907). Journal of the Royal Microscopical Society. p.501.

14 thoughts on “Euparal Mounting Medium”

  1. What can be do if you have the Euparal completely solidified in the bottle? Just adding some drops of Euparal Essence to the bottle would be enough?
    Thanks in advance!

    1. For Euparal there is a specific solvent called “Euparal essence”, which can be bought separately. I never used this, however. I always added a few drops of pure alcohol to the bottle, and then mixed. This is an advantage of Euparal over other mounting media: you do not need xylene or other organic solvents.

    1. Hello,
      Try to contact chemical supplies companies that deliver products to laboratories.
      In Google, type in the following for searching: euparal (spacing is important)
      This will only find Brazilian sites.

  2. Thank you for the good review of the media.
    I would like to add a comment, though – in preparing insect slides, I found that going straight from 70% to euparal can induce bubbles while the resin is curing (say, on a slide heater). The specimens have to go to 100% ethanol…

  3. I have a question: When we move the specimens from the clearing agent (Nesbitt medium) to the Euparal, several bubbles are formed and the Euparal becomes less translucient and emulsify. What can I do to avoid this problem?

    1. Hello,

      The formation of bubbles does not surprise me, because Nesbitt’s fluid is a water-based substance. Euparal is hydrophobic, and the two substances do not mix. The bubbles are most likely water droplets that do not mix with the Euparal.

      You must dehydrate the specimen first, before mounting it with Euparal. Place the specimen into increasing concentrations of alcohol (ethanol), and then last preferably into isopropanol. Even concentrated ethanol does contain a bit of water (which is not a problem) but isopropanol is quite water-free. The step-wise increase of the alcohol concentration prevents the water from being withdrawn too quickly and therefore prevents a shrinkage of the specimen. After all of the water has been removed, you then can transfer the specimen from concentrated alcohol or isopropanol directly into Euparal. Gerlach (1984) recommends that one places the specimen into 30%, 50%, 70%, 90% and 100% ethanol for dehydration. The duration depends on the specimen, but should be a few hours. If you do not want to do the dehydration, then you should use a water-based mounting medium, such as glycerine jelly (glycerol gelatine), or other commercially available water-based mounting media (of which I have no experience, however).


  4. If you use xylene or toluene you may have problems taking your specimens straight from alcohol as any water present will emulsify. Absolute isopropanol is best.

  5. I ordered some Euparol and it is very viscous and stringy. Can I dilute it some? Do I use Xylene? Thank you,

    1. You can use Euparal Essence to thin it down. This is the recommended solvent. I also heard that isopropanol (isopropyl alcohol) should work, but have not experience with this. I try to avoid xylene, due to health reasons, but do not know if it works.

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