Before ultrathin sectioning a sample with an ultramicrotome it has to be pre-prepared. For this pre-preparation, special attention must be paid to the sample size (size of the section), location of the sample (targeting) and accuracy of the block-face edges. This process is generally called trimming, where in the sample is shaped mainly to a frustum of a pyramid.
For instance, to achieve the optimum sectioning results of biological embedded samples, the surrounding embedding material has to be entirely removed. Thus, only the sample is present in the block-face and a "homogeneous" material can be sectioned. Especially when sectioning with a glass knife, the embedding material would influence the section results due to the different compression behaviour. Additionally a further effect on the sectioning results is seen in the edges of the block-face. Only two exact parallel trimmed edges produce a straight ribbon of sections. Furthermore, the sharpness of the trimmed edge has a direct relationship to the section thickness which can be achieved. Finally, a rule of thumb should be mentioned: "The smaller the block-face is trimmed, the easier the sectioning performance will be."
Conventional trimming of embedded samples involves the use of razor blades and a great deal of skill by the ultramicrotomist. The shape of the block face and the straightness of the edges of the trimmed sample has a profound effect on the sectioning characteristics. Parallel edges top and bottom are a must. To achieve this quickly and safely is challenging and potentially hazardous with a razor blade especially on industrial materials. Important specimens can be destroyed by a minor inaccuracy in trimming.
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