Print Friendly, PDF & Email

Storage Containers for Fluid-preserved Specimens


The criteria for a good fluid storage container are high material quality and stability (non-reactive), effective closure and seal, and, good price and availability. Because no single type is perfect in all regards, some compromise of these criteria is necessary. Pros and cons of various containers and lids are presented here, as well as modifications to improve characteristics.


Arnold Y. Suzumoto
Zoology Department
Bernice Pauahi Bishop Museum
Box 19000-A
Honolulu, HI 96817 USA
Tel (808) 848-4115
Fax (808) 841-8968

Figures 1, 3, 4, 6:
Arnold Y. Suzumoto
Figures 2, 5: Julio Gisbert and
Rosa García-Perea

Publication: 1992



Glass containers

Of the glass storage containers reviewed, the best sealing is the screw-top jar with a polyethylene insert or liner and a polypropylene lid (Fig. 1). Next, practically equal in efficacy, are the cam-lock jars and bail-top jars, especially if the rubber gaskets are in good condition and the retaining wire used is of durable quality. Next is the wide-mouth gallon jar with a two-piece plastic lid, especially if a sheet of polyethylene terephthalate (PET) or polyethylene is utilized as a liner. Glass crocks are useful because of their size, and seal fairly well if a gasket of neoprene foam or polyethylene foam is substituted for the hard rubber gasket usually associated with these.

Figure 1. Screw-top glass jars. Each jar comes with a polyethylene insert and a polypropylene lid.

Non-glass containers

Of the non-glass containers, perhaps those of polypropylene used for food storage work best in containing fluids. It remains to be seen how effectively these containers perform in the decades to come. These and polyethylene terephthalate jars, have a generally higher initial cost, and the use of plastic containers replaces products based on abundant silica with those based on petroleum resources.

Final considerations of cost (including shipping and international customs fees, if applicable) and product availability will dictate whether or not any given type of container is ultimately purchased. A hidden cost to consider, too, is the number of person-hours required to add fluid to containers of low initial costs, but of such design that allows evaporation. The practice of topping-off also changes the micro-storage environment. Until such change and its effects on jar contents can be evaluated, it would be prudent to consider topping-off frequency as a major negative factor in container selection.


Stainless steel tanks and wooden tanks with fiberglass resin coatings are effective if the seams are well inspected and tested and if their gaskets are of suitable material. Fiber barrels work effectively for several years, but long-term effectiveness is questionable.

Materials Tools Supplies

  • Glass jars with polypropylene lids and polyethylene inserts
  • Polyethylene foam liners for lids or polyethylene sheet disks
  • Polyethylene terephthalate film, 4mil
  • Scissors

Materials Tools Supplies

  • 110-115volt AC power source
  • Adjustable retort stand with adjustable platform constructed of 12mm plywood for holding plastic during bending
  • Bending jig constructed of 12mm thick plywood
  • Buffing wheel
  • C-clamps, 10in
  • Chloroform (Hazardous material threshold limit value:10ppm – see precautions under comments.) or methylene chloride 
    (see precautions under comments.)
  • Cover slip for glass slides
  • Electric drill
  • Furniture clamps (various lengths)
  • Heat resistant pads
  • Jewelers rouge
  • Lead weights
  • Masking tape
  • Sand paper, 100-600 grit
  • Solid core heater bar (variable lengths) manufactured from low-resistance steel alloy
  • Syringe (50-100 ml) and large bore needle (13-15 gauge)
  • T-bar holding device with adjustable arm for securing plastic during cooling
  • Table or radial arm saw
  • Transparent polymethylmethacrylate plastic sheeting, various thicknesses from 3-12mm (determined by jar volume)


  1. Purchase or cut polyethylene liners.
  2. Replace metal and Bakelite lids with polypropylene lids, with polyethylene liners, or at least, remove pulp liners and replace with polyethylene inserts or liners.
  3. Insert a sheet of polyethylene terephthalate film between the jar and lid where needed.


Jars – Glass

Ground glass jars

The “ground glass jar” incorporates a carefully matched glass stopper with sealing surfaces ground to fine roughness, mated with a glass vessel with finely ground surfaces on its inside neck (Fig. 2). The problem with ground glass jars is that the mated stopper and jar are often separated, in cleaning procedures, for example, and never appropriately matched thereafter; or, the mass-produced versions of these don’t fit well enough without some sealing compound applied. Usually these sealing greases or waxes do not offer adequate long lasting seals and may contaminate the preservative fluid, or be solubilized by it, allowing rapid evaporation.

Figure 2. Ground glass jars.

Another problem with ground glass jars is that the pieces often stick, or lock together, making disassembly nearly impossible without breaking the vessel and/or possibly injuring the handler.

When made of high-quality borosilicate glass, with carefully matched ground glass stoppers, however, these jars provide a very good storage container. (See P. Clark, “Ground Glass Stoppered Jars for Fluid Collections,” this volume.)

Cam-lock and bail-top jars

“Cam-lock jars” (or “French canning jars”) and “bail-top jars” both incorporate sealing devices of heavy gauge wire and cam mechanisms to lock down all-glass lids against a rubber or polychloroprene (synthetic rubber) gasket (Fig. 3).

Figure 3. Bail-top jars.

These jars are very effective in sealing fluids, but the gasket may be susceptible to alcohol and formaldehyde attack. Polychloroprene may contaminate (darken) alcohols (the effect of this on specimens remains unknown).

Improved synthetic rubber compounds made of an acrylonitrile-butadiene copolymer are better. Silicone gaskets that are supplied with some cam-lock jars may not be effective in stopping the flow of alcohol molecules.

Glass crocks

“Glass crocks” are all-glass, wide-mouth containers that are sealed with a metal clamp, with a turn-screw providing sealing pressure against the lid (Fig. 4). “Band-top jars” are usually of smaller capacities than most glass crocks, and usually have much narrower mouths. A thin metal band fits across the top of the lid and acts to clamp it to the jar. Both jar types seal with a gasket of rubber or similar material and are subject to the general failings of gaskets.

Figure 4. Glass crocks sealed with turn-screw metal clamp.

Copenhagen jars

“Copenhagen jars” (or “Danish jars”) are wide-mouth glass vessels with snap-on polyethylene lids. These remain effective containers so long as the polyethylene lids remain intact. Some users report that the lids split after a period of time.

Jars – Plastic

Polypropylene containers are suitable for fluid storage if seals are adequate.

Containers of crystal-clear polyethylene terephthalate (PET), are currently available. Lower in weight than glass, PET jars may save on shipping expenses and may be a minor consideration in floor weight loading. Breakage is not a problem, although UV embrittlement may be. Oxygen permeance could be a problem as well.

Metal lids

Metal lids are usually made of steel and are often plated with brass inside and painted with black or white (sometimes clear) enamel outside. All eventually corrode. Rusting from the inside out is especially severe for jars containing formaldehyde solutions. Dust settling on the lid in high humidity creates conditions for external rust to form, especially if the external enamel coat is scratched. 

Metal lids do not loosen as readily as Bakelite lids, but the cardboard liners are subject to compressive set and shrinkage, leaving gaps which allow evaporation. The performance of metal lids may be improved by replacing the pulp liners with die-cut polyethylene liners. However, metal lids should be replaced by polypropylene lids.

Plastic lids

Bakelite resin

Bakelite lids are rigid and almost always black, with a smooth, glossy or matte surface (Fig. 5). Bakelite fails to provide a reliable closure for formaldehyde and alcoholic solutions because the vapors embrittle the resin. Also, Bakelite has a different expansion/contraction rate than glass, so any temperature variations over time will lead to loosening of the lid. No matter how tightly a lid is applied, eventually (and it could be as quickly as a month with a 3°C flux around an average of about 26°C) the lid will loosen to the extent that fluids will evaporate.

Figure 5. Glass jars with Bakelite lids.

The performance of Bakelite lids may be significantly improved by using a polyethylene insert or disk liner. 

Inserts may be purchased in 80mm and 40mm diameter sizes, or disks may be cut from polyethylene sheets or foam. Slight trimming of the insert edge and removal of the liner from the Bakelite lid are required for a good fit.

Polypropylene and polypropylene/polyethylene lids

Polypropylene is slightly lighter than water; similar looking lids of styrene or other hard plastics will sink in water. Different colored polypropylene lids are available. The flexibility of polypropylene allows it to expand and contract with glass without much loosening. Polypropylene is subject to ozone and UV attack, but such conditions should be minimal in most collections. The associated liners are usually of polyethylene, either solid disk or foam disk. A smooth-surfaced foam disk is thought to perform the best, compressing against the jar mouth without significant abrasion damage to the liner.

A two-piece plastic lid supplied for wide-mouth gallon jars utilizes two long-lasting, ideal polymers: the disk is polyethylene; the securing ring is polypropylene. The materials will last indefinitely in most collections, but there are reports of minor discoloration (yellowing) due to presumed UV exposure. Of greater concern is the fact that, whichever way the disk is positioned on the jar (smooth side in or smooth side out), leaks occur. A sheet of polyethylene terephthalate film or polyethylene sheeting between jar and lid usually solves most leakage problems.

Translate »