Monthly Archives: November 2015

Fire Extinguishers: Demystified

Types of Fires and Fire Extinguishers

There are four main types of fire:

  1. Class A: Ordinary Combustibles, such as wood, paper, and cloth
  2. Class B: Flammable Liquids and Gases, such as oil, gasoline, propane, and paints
  3. Class C: Live Electricity, includes electrical equipment
  4. Class D: Combustible Metals and Metal Alloys, such as magnesium, titanium, sodium and potassium (this type of not very common)

Every extinguisher is labelled to indicate which types of fires the extinguisher is effective against, using the fire type classification system. For example, a fire extinguisher labelled ABC is effective against class A, B and C fires. There may also be numbers in front of the class designations. These numbers indicate the comparative effectiveness of the extinguisher to combat the specified type of fire. Higher numbers indicate higher levels of effectiveness. For example, a fire extinguisher labelled 1-A:10-B:C is less effective at combatting class A fires than a fire extinguisher labelled 2-A:10-B:C. Class C does not have a number classification.

Conduct a risk assessment to identify your fire hazards and ensure you purchase fire extinguishers that are effective against potential fires at your archives, be it class A, B, C or D or a combination of fire types.

Extinguishing Agents

There are different extinguishing agents for each class of fires:

  1. Water extinguishers: Class A fires only. Never use water on class B, C or D fires. Water may cause damage to archival records and electronics.
  2. Dry chemical extinguishers (powder with pressurized nitrogen): Combination of class A, B, and C fires (check label of extinguisher). The residue may cause damage to archival records and electronics.
  3. Chemical foam (aqueous film forming foam): Class A and B fires only. The residue may cause damage to archival records and electronics.
  4. Compressed gas extinguishers (pressurized CO2 or halon): Class B and C fires only. This does not leave a harmful residue, but is not effective on class A fires.

It is extremely important to know the type of fire extinguisher you are using and what type of fire you are dealing with. Using the wrong extinguishing agent may feed the fire instead of suppressing it. If you do not know the type of fire, the type of extinguisher or how to use it, evacuate the building safely and call the fire department.

It is important to research the effects of these extinguishing agents on the media types and equipment found in your facility. Some of ASA’s institutional members have installed CleanguardTM Clean Agent Extinguishers at their facilities**. CleanguardTM uses DuPontTM FE-36TM as the extinguishing agent. The producers claim it is electrically nonconductive, low in toxicity, and environmentally friendly, in addition to being an effective fire suppressor. Conduct research on all types of extinguishing agents and their effects on your collection and equipment prior to purchasing any products.

** The ASA does not endorse the use of any particular brand of fire extinguisher.

Where to House Fire Extinguishers

It is recommended to house fire extinguishers near potential hazards identified in your risk assessment. They should be housed visibly near an exit. It is recommended to have your back to an exit while extinguishing fires in order to have a clear escape route. For example, a staff kitchen is usually identified as a fire risk in a risk assessment. If possible, store the fire extinguisher on the wall adjacent to the door to the kitchen.


Fire extinguisher training is extremely important. Do not attempt to operate a fire extinguisher without the proper training. Consider offering training to your staff as part of your disaster preparedness training. This may be offered through your Occupational Health and Safety Department or your local Fire Department.

Here is an acronym to help you remember how to use an extinguisher once you have received training:

Pull the fire extinguisher’s safety pin.

Aim at the source or base of the flames. It is recommended to stand at least 6 feet away from the flames; however, the fire extinguisher should indicate how far away you should stand from the flame.

Squeeze the handle or trigger.

Sweep the source or the base of the flames until the extinguisher is empty.

After Using a Fire Extinguisher

If you use a fire extinguisher, refill or replace it immediately. Extinguishers with metal valves can be refilled and extinguishers with plastic valves cannot be refilled and should be discarded and replaced.

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Could it be Cellulose Nitrate?

When completing a collection survey, you may come across negatives and be uncertain of whether or not they should be frozen. There are many ways to determine what the base of the negatives could be, including various chemical tests and burning tests. Purchasing chemicals can be expensive, and dangerous to house and dispose of. While burning tests work, they require sacrificing a piece of the negative and lighting a fire.

A safe, reusable, environmentally friendly, and inexpensive method of analyzing negatives is to look at one between two linear polarizers. Linear polarizers are a type of grey, transparent film used in photography as a camera filter. When purchasing linear polarizer film, it is important not to order circular polarizer film as it will not perform the same way.

How to use it:

  1. Cut two pieces of film from a sheet.
  2. Create a frame for both pieces using acid-free mat board. This will help protect the film from abrasions and fingerprints.
  3. Sandwich the negative between the two pieces of matted film and hold up to natural light.
  4. Rotate one piece of matted film.

What to look for: When the one of the matted film pieces is rotated you will see one of two things: The negative and film will both appear to darken, or colours will be seen, called birefringence.

Birefringence. Photo credit: Dave See

Birefringence. Photo credit: Dave See

Birefringence will only be encountered when looking at a polyester negative. If you see the film darken, the negative is either nitrate or acetate. Polyester negatives are relatively stable and do not need to be frozen. Cellulose nitrate and cellulose acetate negatives should be frozen as they off-gas nitrous oxide and nitrogen dioxide or acetic acid, and they rapidly degrade. Cellulose nitrate is also a fire hazard because it can auto-ignite, and the fire cannot be put out.

To further identify between cellulose nitrate and cellulose acetate, you can consult the Northeast Document Conservation Center’s guide to visually identifying negatives:

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