US MK1 AC Delay Firing Device (Chemical Delay Firing Device, M1)
Overview
The US MK1 AC Delay Firing Device, more commonly designated in US Army nomenclature as the Firing Device, Demolition, M1, Delay, is a chemical time-delay initiating device used to fire demolition charges, mines, and booby traps after a predetermined delay period. The “AC” designation refers to the Acetone-Celluloid (or more accurately, acid-corrosive) delay mechanism, which was adapted from British designs developed for the Special Operations Executive (SOE). This device provides delay times ranging from approximately 6 minutes to over 18 hours, allowing personnel to initiate demolition operations and withdraw to safety or establish alibis for covert sabotage missions.
Country/Bloc of Origin
- Country of Origin: United States of America
- Design Lineage: Adapted from British “Time Pencil” technology developed by SOE
- Developing Organization: US Office of Strategic Services (OSS) Research & Development; later standardized by US Army Ordnance
- Development Period: 1942-1943
- Collaborative History: The US version was developed in coordination with British intelligence services; American chemist Stanley Lovell of OSS R&D refined the British design for mass production
- Production: Manufactured by multiple US contractors; millions produced during WWII
- International Use: Supplied to Allied forces and resistance movements throughout Europe and Asia
Ordnance Class
- Type: Chemical Delay Firing Device / Time-Delay Initiator
- Primary Role: Delayed initiation of demolition charges, mines, and booby traps
- Delivery Method: Hand-emplaced
- Category: Initiating and Priming Device
- Function: Provides chemically-timed delay detonation for sabotage, demolition, and booby trap applications
Ordnance Family/Nomenclature
Official US Army Designations:
- Firing Device, Demolition, M1, Delay
- Chemical Delay Firing Device M1
OSS/SOE Terminology:
- Time Pencil (British origin term)
- AC Delay (Acetone-Celluloid/Acid-Corrosive)
- MK1 AC Delay
DODIC (by delay time, color-coded):
| Delay Period | Color Code | DODIC |
|---|---|---|
| 6-14 minutes | Black | M616 |
| 12-32 minutes | Red | M619 |
| 45-115 minutes | White | M620 |
| 100-280 minutes | Green | M621 |
| 210-570 minutes | Yellow | M622 |
| 610-1130 minutes | Blue | M623 |
| Mixed Pack | Various | M617 |
Related Family Members:
- British Switch No. 10 Delay: Original British design using cupric chloride
- British Switch No. 9 L-Delay: Lead-creep mechanical delay variant
- British AC Delay Mk I: Acetone-celluloid British variant
- US M1 Pull-Type Firing Device: Mechanical pull-activated device
- US M1 Release-Type Firing Device: Weight-release activated device
- US M5 Pressure-Release Firing Device: Pressure-release activated device
Common Names/Nicknames:
- Time Pencil (from British usage)
- Delay Pencil
- Chemical Delay
- AC Delay
- Acid Delay Fuse
Hazards
Primary Hazards:
- Corrosive Chemical Hazard: Contains glass ampoule filled with corrosive liquid (cupric chloride or similar acid compound); causes chemical burns on skin contact
- Detonation Hazard: Once activated, device will eventually fire; attached blasting cap will detonate
- Unpredictable Timing: Delay times vary significantly with temperature; may fire earlier or later than expected
Sensitivity Considerations:
- Crush Sensitivity: Squeezing the copper tube breaks the internal glass ampoule—avoid premature crushing
- Temperature Sensitivity: Chemical reaction rate doubles approximately every 10°C (18°F) increase; halves with equivalent decrease
- Impact: Rough handling may break internal ampoule prematurely
- Blasting Cap: Contains sensitive primary and secondary explosives
Environmental Stability:
- Temperature dramatically affects delay time:
- At 32°F (0°C): Delays may be 2-3 times longer than nominal
- At 90°F (32°C): Delays may be 50% shorter than nominal
- Moisture ingress can dilute corrosive liquid and extend or prevent functioning
- Extended storage degrades corrosive liquid effectiveness
Special Hazards:
- No Abort Capability: Once the ampoule is crushed, there is no way to stop the device from firing (except disposal by EOD)
- Silent Operation: No indication of countdown; no noise or visible signs of activation
- Unpredictable Failure Mode: May fire prematurely if ampoule breaks during handling
Danger Areas:
- Danger radius depends on attached explosive charge
- Minimum safe distance for standard demolition charges: 300 meters
UXO Considerations:
- Chemical delay devices with missing safety strips should be considered armed and dangerous
- Corroded or aged devices may have unpredictable functionality
- Never crush or manipulate suspect devices
- Report to EOD personnel for disposal
Key Identification Features
Dimensions:
- Overall Length: Approximately 5-1/4 inches (133mm)
- Diameter: Approximately 5/8 inch (16mm)
Physical Characteristics:
- Two-piece tubular body construction
- Upper section: Thin copper tube (crushable, contains glass ampoule)
- Lower section: Brass tube (contains firing mechanism and coupling base)
- Color-coded safety/identification strip indicating delay time
- Primed coupling base with protective cap at bottom
- Inspection hole near safety strip location
Color Coding (Safety Strip Colors):
| Color | Nominal Delay Time |
|---|---|
| Black | 6-14 minutes |
| Red | 12-32 minutes |
| White | 45-115 minutes (approximately 1-2 hours) |
| Green | 100-280 minutes (approximately 2-5 hours) |
| Yellow | 210-570 minutes (approximately 3.5-9.5 hours) |
| Blue | 610-1130 minutes (approximately 10-19 hours) |
Distinctive Features:
- Thin-walled copper section (upper portion) that is designed to be crushed
- Color-coded identification strip passing through the device
- Inspection hole to verify striker has not released
- Standard threaded coupling base for attachment to explosive charges
Material Composition:
- Upper tube: Thin copper (for crushing)
- Lower tube: Brass
- Internal components: Glass ampoule, corrosive liquid, tension wire, spring-loaded striker
- Coupling base: Metal with percussion primer
- Blasting cap: Standard M7-type nonelectric blasting cap
Fuzing Mechanisms
Operating Principle: The M1 Chemical Delay Firing Device operates on the principle of chemical corrosion of a restraining wire. A glass ampoule containing corrosive liquid is housed in the copper tube section. When crushed, the liquid releases and begins corroding a thin metal wire that restrains a spring-loaded striker. When the wire is eaten through, the striker is released and driven into a percussion primer, which initiates the attached blasting cap.
Internal Components (from top to bottom):
- Copper tube section: Contains sealed glass ampoule with corrosive liquid
- Glass ampoule: Contains cupric chloride or similar corrosive chemical
- Restraining wire: Runs along the ampoule and into the firing mechanism
- Spring-loaded striker: Held under tension by the restraining wire
- Identification/Safety strip: Blocks striker from hitting primer
- Percussion primer: Initiates blasting cap when struck
- Coupling base: Threaded base with attached blasting cap (or nipple for attaching cap)
Activation Sequence:
- Preparation:
- Remove protective cap from coupling base
- Attach blasting cap to coupling base nipple (if not pre-attached)
- Insert coupling base into explosive charge capwell or attach to detonating cord
- Activation:
- Squeeze/crush the copper tube section to break the internal glass ampoule
- This can be done with pliers, boot heel, or hand pressure
- Complete crushing is not required—just enough to break the ampoule
- Verification:
- Look through the inspection hole to verify the striker has not been released
- If the hole is clear (you can see through), the device is functioning properly
- If the hole is blocked, the striker has already released—discard the device
- Arming:
- Remove the color-coded safety/identification strip by pulling
- The device is now armed—chemical corrosion has begun
- Functioning:
- Corrosive liquid gradually eats through the restraining wire
- Delay time depends on liquid concentration and ambient temperature
- When wire parts, spring drives striker into primer
- Primer ignites, initiating blasting cap and explosive charge
Critical Notes:
- Irreversible: Once the ampoule is crushed and safety strip removed, functioning cannot be stopped
- Silent: No audible or visible indication of countdown
- Variable: Delay times are approximate and highly temperature-dependent
Safety Features:
- Color-coded safety strip blocks striker until removed
- Inspection hole allows verification of striker status
- Glass ampoule protected by copper tube until deliberately crushed
History of Development and Use
Origins and Development:
The chemical delay firing device concept originated in Britain prior to World War II. The British Special Operations Executive (SOE) developed the “Time Pencil” or “Switch No. 10 Delay” at Station XII (Aston House) in Hertfordshire. The British design used cupric chloride to corrode a restraining wire.
When the United States entered the war, the Office of Strategic Services (OSS) recognized the need for similar devices. OSS Research & Development, under the direction of Stanley Lovell, refined the British design for American mass production. The resulting device was standardized as the Firing Device, Demolition, M1, Delay.
Development Timeline:
- 1939: British Section D (SOE predecessor) develops initial chemical delay mechanisms
- July 1940: SOE established; continues development at Station XII
- 1941-1942: US enters war; OSS begins developing American version
- 1942-1943: US M1 Chemical Delay standardized and enters mass production
- 1943-1945: Millions produced; supplied to Allied forces and resistance movements
- Post-WWII: Remains in US inventory as contingency item
- Present: Classified as Contingency (CON); largely replaced by modern systems
Design Philosophy: The chemical delay was chosen because it:
- Operates silently (no ticking like mechanical timers)
- Requires no batteries or external power
- Allows very long delay times (hours to nearly a full day)
- Is compact and lightweight
- Is difficult to detect with standard bomb detection methods
Combat and Covert Use:
- SOE/OSS Sabotage Operations:
- Factory sabotage in occupied Europe
- Railway demolition missions
- Supply line interdiction
- Allowed agents to establish alibis before detonation
- Famous Operations:
- St. Nazaire Raid (1942): British Time Pencils used in HMS Campbeltown’s demolition charge
- July 20 Plot (1944): German conspirators used captured British Time Pencils in attempt to assassinate Hitler
- Numerous SOE and OSS sabotage missions throughout occupied Europe
- Resistance Movements:
- French Resistance, Polish Home Army, and other groups used Allied-supplied chemical delays
- Factory workers used short-delay versions to sabotage war production
Temperature Reliability Issues: One documented issue was temperature sensitivity. The St. Nazaire Raid demonstrated this when HMS Campbeltown’s demolition charge detonated over an hour late due to cooler-than-expected temperatures. This reinforced the practice of using two devices from different batches for critical missions.
Current Status:
- Contingency (CON) classification in US inventory
- Largely replaced by electronic delay detonators and shock tube systems
- May remain in stocks of some nations
- Historical significance in special operations and resistance warfare
Production Numbers:
- British: Approximately 12 million Time Pencils produced during WWII
- US: Several million M1 Chemical Delay devices produced
- Combined Allied production supported global resistance operations
Technical Specifications
| Specification | Details |
|---|---|
| Overall Length | Approximately 5-1/4 inches (133mm) |
| Diameter | Approximately 5/8 inch (16mm) |
| Weight | Approximately 1.5 oz (42g) |
| Body Materials | Copper (crush tube), brass (firing mechanism) |
| Housing Color | Olive drab with yellow markings |
| Delay Mechanism | Chemical corrosion of restraining wire |
| Corrosive Agent | Cupric chloride or similar acid compound |
| Output | Standard M7-type blasting cap |
| Quantity-Distance Class | 1.4 |
| Storage Compatibility Group | S |
| DOT Classification | Class C Explosive |
| DOT Designation | PERCUSSION FUZES |
| UN Number | 0349 |
| UN Shipping Name | Articles, explosive, n.o.s. |
Delay Time Specifications:
| Color Code | Nominal Delay | Temperature Variation |
|---|---|---|
| Black | 6-14 minutes | ±50% with temperature |
| Red | 12-32 minutes | ±50% with temperature |
| White | 45-115 minutes | ±50% with temperature |
| Green | 100-280 minutes | ±50% with temperature |
| Yellow | 210-570 minutes | ±50% with temperature |
| Blue | 610-1130 minutes | ±50% with temperature |
Temperature Effects:
- At 90°F (32°C): Delays approximately 50% of nominal
- At 60°F (15°C): Delays approximately at nominal
- At 32°F (0°C): Delays approximately 200-300% of nominal
- Below freezing: May fail to function or have extremely long delays
Packaging:
- 10 devices per setup box
- Boxes vapor barrier bagged
- 15 bagged boxes per carton
- 3 bagged cartons (450 devices) per wire-bound wooden box
- Box weight: Approximately 56 lbs
Frequently Asked Questions
Q: Why is this device called an “AC Delay” device? A: The “AC” designation historically refers to Acetone-Celluloid, describing the original British mechanism where acetone dissolved a celluloid disc. The US version and many British variants actually used cupric chloride (or similar acid) corroding a metal wire, but the “AC” terminology persisted. In practice, “AC Delay” became a general term for chemical time-delay initiators regardless of the specific chemistry employed.
Q: How accurate are the delay times? A: Chemical delay times are approximate and highly variable. A device rated for a 2-hour delay might actually fire anywhere from 1 to 4 hours depending on temperature. This is why critical missions used two devices from different production batches—the variance ensured at least one would function within an acceptable window. For precise timing, modern electronic delay detonators are far superior.
Q: What happens if the ampoule breaks accidentally during handling? A: If the ampoule breaks before deliberate activation, the corrosive liquid immediately begins attacking the restraining wire. If the safety strip is still in place, it provides a backup by blocking the striker. However, the device should be considered compromised and disposed of by EOD. Never attempt to use a device that may have been accidentally activated.
Q: Can the delay be stopped once the device is activated? A: No. Once the ampoule is crushed and the safety strip removed, there is no way to stop the chemical corrosion process. The only safe response is to evacuate the area and allow the device to function, or have EOD personnel remotely dispose of the device and attached explosives. This irreversibility was actually a security feature—captured agents could not reveal device locations in time to prevent detonation.
Q: Why were chemical delays preferred for sabotage operations? A: Chemical delays offered several advantages for covert operations: they operate silently (unlike ticking clockwork), require no batteries, allow very long delays permitting agents to establish alibis or escape, are compact and easily concealed, and were difficult to detect with period bomb detection technology. The unpredictable timing actually provided additional security—enemy bomb disposal teams couldn’t predict when detonation would occur.
Q: How do I select the correct delay time for an operation? A: Consider the mission requirements, ambient temperature, and safety margin. For hot conditions, select a longer nominal delay than needed. For cold conditions, select a shorter nominal delay or consider alternative initiation methods. For critical missions, use two devices—one slightly longer and one slightly shorter than the target time—to ensure detonation within an acceptable window.
Q: What safety precautions are essential when using the M1 Chemical Delay? A: Always handle the copper tube section gently until ready to activate. Verify the inspection hole is clear before removing the safety strip. Account for temperature effects on timing. Use two devices from different batches for critical missions. Never remain in the danger area—treat the device as if it might fire at any moment after activation. Never attempt to disassemble or reset an activated device.
Q: What replaced the M1 Chemical Delay in modern military use? A: Modern operations use electronic delay detonators with precise programmable timing, shock tube systems (NONEL) with reliable delay elements, and radio-controlled firing devices. These systems offer accurate timing, remote initiation capability, and enhanced safety features. However, chemical delays remain relevant as contingency items because they require no batteries and are difficult to detect or jam.
This lesson is intended for educational and training purposes. All ordnance should be considered dangerous until proven safe by qualified personnel. Unexploded ordnance should never be handled by untrained individuals—report findings to military or law enforcement authorities.