Artefacts (or artifacts) are human made items or the results of human activity. Fixation artefacts are consequently changes brought about in the tissue as a result of human activity, i.e. putting some tissue in a fixative.
From that definition, fixation itself could be considered to be an artefact, as it is a change brought about by human activity, and sometimes that comment is made. It is certainly true that fixation alters the tissue state: it ceases to be a dynamic system and becomes static, with most chemical processes and physical structures suspended in the state they were when the fixative's chemicals encountered them. Despite that, a lot can be determined by examining that snapshot in time which is represented by a microscope section.
More particularly however, artefacts are usually defined as the unwanted effects of a process, including fixation. In practice these unwanted effects are largely restricted to a few items such as deposits from the fixative or its reactions with tissue components, and the physical effects of fixation we would prefer to do without, such as detectable shrinkage, both at the gross and microscopic levels; the hardening of some tissues which causes chatters parallel to the knife edge; and the brittleness from some fixatives which results in shattering and cracking during sectioning.
There are two deposits in particular that we may encounter fairly often. Formalin pigment, or acid formaldehyde hematein, is frequently seen as fine granules, particularly in bloody tissues, and mercury pigment is invariably encountered in tissues fixed with mercuric chloride. Both are easily removed.
Although, strictly speaking, it is not a fixation artefact, the removal of triglycerides – fats – is a consequence of the failure of the fixative to render them unaffected by subsequent reagents. Most fixatives are deficient in this area, and blank holes are often seen where there was, in life, fat globules.
Not too many years ago a new artefact began to be mentioned, and was named the parched earth effect. It quickly became apparent that this was not a previously undetected artefact, but was, in fact, the undesirable effects of warm ethanol fixation. It began to be seen when more laboratories switched to processing times of less than 24 hours from biopsy to diagnostic report. This can only be done with paraffin sections if each step is cut to the absolute minimum. Unfortunately, many laboratories did not give adequate time for the most fundamental step, fixation, to be adequately effective and protect the tissue. The result was that fixation was completed in subsequent steps with (warmed) ethanol. Parched earth artefact, then, is just the poor quality fixation obtained with ethanol, exaggerated by applying it warm to speed up dehydration.