Euparal is a semi-synthetic mounting medium used in microscopy. It 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 , published in French. An article published in the Journal of the Royal Microscopical Society  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.