Chemistry of Embalming Fluids, Preservatives and Additives
Chemistry of Embalming Fluids
Formaldehyde: the molecule
Formaldehyde (CH2O) is a gas; the solution supplied is formalin, roughly 37-40% formaldehyde stabilised with methanol. In water formaldehyde hydrates to methylene glycol; in tissue it cross-links protein amino groups, hardening and fixing the proteins. Strength of the concentrate is expressed as the index: grams of formaldehyde per 100 mL. An index of 20 means 20 g per 100 mL.
Dilution arithmetic
The working strength is what preserves tissue, not the number on the bottle. Use C1V1 = C2V2, and watch the trap: adding one gallon of fluid to two gallons of water makes three gallons, so the index is reduced to one third, not one half.
Worked example: to make 4 litres of a 2% solution from an index-20 fluid, volume of concentrate = (2 / 20) x 4000 mL = 400 mL, made up to 4 litres with water.
Other chemistry
pH matters: formaldehyde solutions are acidic and are buffered towards neutral to reduce swelling and roughness. Hard water contains calcium and magnesium that consume preservative and cause cloudiness, so water conditioners are added. Concentrated formaldehyde polymerises to paraformaldehyde in the cold, producing a haze and reducing effective strength.
Traps and a rule of thumb
Index confused with working percentage; stronger is assumed to be always better; additives are assumed to be inert. Rule of thumb: read the label, calculate in millilitres, and check the arithmetic twice.
Preservatives and Additives
The main preservative
Formaldehyde is the primary arterial preservative - bactericidal, virucidal and tuberculocidal, and a powerful fixative. It is not reliably sporicidal or active against prions and is largely fungistatic. Glutaraldehyde is a stronger, less volatile alternative; phenol is used mainly in cavity fluids; alcohols are weak preservatives but useful solvents.
The supporting cast
Buffers (borax, sodium citrate) hold pH. Anticoagulants (sodium citrate, sodium oxalate, EDTA) prevent clotting and improve drainage. Humectants (glycerin, sorbitol, lanolin) retain moisture for a natural feel. Surfactants and wetting agents (sodium lauryl sulphate) lower surface tension so fluid penetrates. Salts (sodium chloride, sodium sulphate) influence osmotic balance and swelling. Dyes (eosin, erythrosine, amaranth) restore colour. Water conditioners (EDTA, trisodium phosphate) deal with hard water, and mould inhibitors and perfumes complete the formulation.
Accessory chemicals
Pre-injection fluid, containing anticoagulant and wetting agent but little or no formaldehyde, clears the vascular bed before the preservative arrives. Co-injection fluid strengthens the arterial solution. Cavity fluid is high-index and often phenolic. Surface packs and hypodermic treatments reach tissue the arteries cannot supply.
Traps and a rule of thumb
Examiners mix humectant with preservative, confuse cosmetic dyes with non-cosmetic tracers such as fluorescein, and forget that excess humectant leaves tissue wet and soft. Rule of thumb: the additive list is a checklist of problems - clotting, hard water, dryness, colour, penetration - and each additive answers one of them.