As Clearing Agents
Some clarification of terms is required due to the differences in meaning in different countries. In this section the term "paraffin" refers to a group of chemicals more accurately referred to as alkanes and related compounds. It does not refer to the heating oil used in the UK and some other countries. Paraffins are a broad group and range from the gaseous, through liquid and oily, to solids.
The simplest alkanes are methane, propane, butane etc, including natural gas, although these are rarely termed "paraffins" in common speech.
The "paraffinic solvents" are low viscosity fluids and are identified as mineral spirits, naphtha, white gas or Stoddard's solvent. The major producers of these fluids are Exxon and Shell. Exxon uses the trade names "Isopar", "Exxsol" and "Norpar" for their products, while Shell uses "Tolu-sol" and "Tolu-sol HT". Some of the products in this group are suitable as clearing agents.
There is also the old fashioned constipation remedy "liquid paraffin". This is an oily fluid, with a viscosity like honey. It has no particular use in histology.
The final group are the solids. These are usually referred to as "candle wax" or "paraffin wax", and are white solids which are easily melted on heating. The melted paraffin wax should be referred to as "molten paraffin" and not as "liquid paraffin", as I have heard occasionally, so as to avoid confusion. Paraffin wax, of course, is used extensively in histology for infiltration and sectioning, although it is usually modified with additives.
In this discussion we are concerned with the "paraffinic solvents" These solvents are mixtures of various alkanes and related compounds, rather than a single chemical, and may include n–paraffins, iso–paraffins and naphthenes, that is, straight chain, branching chain and cyclic alkanes. The three diagrams below illustrate the types of compounds. Left to right they are n-octane, iso-octane and cyclo-octane. However, they are illustrative, not definitive. Solvents provided will be made of similar products, depending on the manufacturer and they may vary significantly in the number of carbons in the molecule and the proportions of each type, and this may affect their viscosity and inflammability.
These solvents are often used in the belief that they are safer than xylene or toluene, and that may be so, but it should not be presumed that thay are completely safe and non toxic. Long term exposure to them, as we may have in a confined area processing room, can have serious neurological effects. It must always be kept in mind, as well, that they are very inflammable, some of them more so than xylene. Precautions must be taken to ensure they can be used safely.
Since this group is so diverse, and since different manufacturers sell similar mixtures there is a great deal of inconsistency in the product as they are produced to meet characteristics established by each manufacturer. This is done by blending components, and the blend may vary from time to time. If used as clearing agents, a purchasing department should not buy from a new supplier without consultation, since the product being purchased may not be the same blend and may behave differently towards the dehydrant. Trials should be run first. In addition, the terminology used for these various solvents appears to be applied quite loosely, particularly in the consumer world. Always check that you are purchasing what you intended to purchase.
Naphtha is available in two broad forms, light or heavy naphtha, the light and heavy referring to density. Both are variable mixtures of the components illustrated above, but light naphtha has a higher content of straight and branched chain compounds and has carbon chain lengths of about 5-9. Heavy naphtha contains more naphthenes and has components with carbon chain lengths of about 7-9. Both are very inflammable
White gas is a type of naphtha used as a fuel in camping stoves or pressure lamps. It is also known as "Coleman fuel" after the brand name of a common camping stove.
Stoddard solvent, also called mineral spirits or white spirit, was originally developed by W. J. Stoddard (hence the name) as a safer alternative to the dry cleaning fluid in use in the first part of the last century. It has compounds with carbon chain lengths of 7-12 and may have up to 25% aromatic hydrocarbons present. It is also very inflammable, but perhaps less so than naphtha. Odorless mineral spirit, is Stoddard solvent which has had the aromatic components removed, thus reducing the smell.
Turps substitute is a mixture of the same components in the range of 9-16 carbons in length. It is used as a replacement for naturally derived turpentine. Its primary use is as a paint thinner and brush cleaner for oil paints.
Kerosene also called paraffin oil in some countries, is a mixture of petroleum components with carbon lengths of 6-16. It is easily available as it is used for heating, cooking and as a jet fuel throughout the world. It has a strong, pervasive odour and sometimes burns with black smoke (carbon). Although it can be used as a clearant it is not recommended due to its odour, and because it may be dirty and require filtering to remove particles that could interfere with sectioning.
Gasoline or petrol is very similar to white gas, but is optimised for use in the internal combustion engine. It may be suitable for use as a clearing agent, but it must be remembered that there may be several additives in the mixture which may cause problems. These could easily include ethanol, the fluid that clearing agents are supposed to remove, since it is becoming much more common to add a proportion of this as a means of reducing unwanted emissions. Such mixtures would be useless for clearing. Only a pure gasoline should be considered. It should also be filtered to remove any dirt which may interfere with sectioning, much like the fuel filter in a car does.
The following Wikipedia articles are worth reading. They give background and chemical information rather than the role as clearing agents.