Table 2

PROBLEM	CAUSES
Crooked or uneven ribbons	Edges of knife and block are not parallel The four sides of the block are not trimmed parallel The knife edge is not regularly sharpened Impurities in the wax causing dense areas Varying densities in the tissue
Wrinkled or compressed sections	Blunt knife Warm block or knife Clearance angle too great Micrometer set too thin for wax hardness Loose knife Bevel of knife too deep making cutting angle too shallow
Thick and thin sections	Loose knife or loose block Block too large for the microtome in use Knife clearance angle too fine Tissue or wax excessively hard Areas of calcification present in tissue
Sections adhere to block on upstroke of microtome	Knife clearance angle too fine Blunt knife Dirty knife edge or dirty underside of block
Ribbons split vertically or scratch lines appear	Small nicks in the knife edge Knife clearance angle too great Hard material embedded in the wax (grit, dirt, crystals, talc) Hard material in the tissue (calcium, crystals, sutures) Dirt on the knife edge
Sections crumble or tear	Tissue is incompletely dehydrated Tissue has not cleared properly Paraffin wax denatured from excessive heating Blunt knife Wrong melting point wax used for the conditions - wax too soft Wax has crystallised from slow cooling Wax is contaminated with clearing agent or water
Sections show areas of varying thickness (chatters)	Loose knife or block Knife clearance angle too great Excessively hard tissue or wax Areas of calcification present within tissue
Sections will not ribbon	Knife clearance angle too fine or too great Dirty knife edge Wax surface too cold
Sections roll up on cutting	Blunt knife Knife clearance angle too great Micrometer set too great Wrong melting point wax for the conditions
Sections disintegrate on water bath	Water temperature too high Wax contaminated with clearing agent