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From: (Gerald L. Hurst)
Newsgroups: alt.engr.explosives,alt.pyrotechnics,rec.pyrotechnics
Subject: Re: Question & answers on detonators
Date: 18 Jan 1996 22:14:44 GMT

In article <4dmc33$>, (Buxton) says:

>I have some questions and answers on the subject of detonators. Please verify 
>my answers. Also feel free to add your own comments on these or related 
>Q.   How does the detonator play a part in explosives? And why should it be 
>     used at all?
>A.   A main charge will not explode merely upon ignition. It needs a "blasted 
>     ignition" to explode.

The secondary charge requires high-pressure adiabatic compression
to generate enough heat over a large enough volume to initiate
a stable detonation which will propagate the pressure/heat in the form
of a so-called shock-wave or pulse.

>Q.   Why should the detonator powder be placed in a tube at all? Why can't 
>     the powder be compacted into the main charge?
>A.   So that no chemical reaction can take place between the detonator powder 
>     and main charge.

Rather, so that the more likely accidental initiation of the primary
charge will not result in a larger accident. Explosives and detonators
are stored separately for this same reason.

Q.   What happens when the detonator powder is ignited?
A.   The basis of detonation works on the principle of shock waves 
     supplemented by ignition. The detonator powder, when ignited, explodes 
     violently and emits large shock waves enough to blast the main charge.

>Q.   What powders are used in detonators?
>A.   HMTD, RDX and PETN are common powders ideally suited for detonators.

Make that PETN and RDX used as base charges, but initiated by a separate
primary explosive. HMTD is a primary explosive and one which is too
unstable for use in blasting caps.  

>Q.   How much powder must be used in relation to the main charge?
>A.   1 part detonating powder to 100 parts main charge.

There is no accepted ratio. It is unimportant as long as there is enough
initiator. A blasting cap can set off an unlimited amount of powder.

>Q.   What size tube must be used for the detonator and how thick should the 
>     wall be?

There is no "must." A military stab detonator may be inly 1/16 inch
in diameter. More typically a detonator is about as wide as a pencil.

>A.   5mm diameter by 50mm long will suit a charge of 100g. The wall should 
>     be as thin as possible.

100 g or 1000 kg. it doesn't matter. Wall thickness is largely 
determined by use. Seismic caps may be designed to withstand
thousands of psi.

>Q.   Why should the powder be compacted into the tube?
>A.   To expel as much air as possible. Air will disrupt the wave emissions 
>     of the exploding detonator powder, thereby decreasing the efficiency of 
>     the detonator substantially.

The secondary explosive in the tube is compressed to at least 1.4
g/cc to ensure adequate detonation pressure, which varies as the
square of the density. Much greater densities may be avoided
to prevent reduction of sensitivity.

>Q.   How much and how hard should the compaction of the detonator powder be?
>A.   As much as the powder will allow.

See previous answer. Excessive pressure must be avoided.

>Q.   In an electrical detonator, why must the detonator powder be separated 
>     by another powder such as Black powder etc. Why can't the ignition 
>     circuit make direct contact with the detonating powder?
>A.   To prevent an unequal combustion force.

Because the secondary powder in modern caps requires a primary
explosive (NOT BP) to initiate it. BP's only use would be as
a delay element. Earlier caps used primary explosives only and
could be initiated by a match composition or BP. There are 
detonators which use only electrical energy to initiate secondary
charges but such are used only by those who have access to powerful 
large capacitor discharge units capable of exploding a metal 
bridgewire with sufficient energy to simulate a chemical detonation.
The Germans use such caps for bridge demolition. Stanford Research
also makes and uses exploding bridgewire caps. 

>Q.   How snug must the detonator be placed into the main charge? Will a loose 
>     fit affect the efficiency of the final explosion?
>A.   Certainly. The more snug the detonator, the better the transfer of the 
>     waves from detonator to main charge. Here the law of shock waves apply.

The fit required depends on the explosive. Dynamite will tolerate
a lot of sloppiness, but plastiique, Kinepak and water-based 
explosives demand some care in placement.

>Q.   Should the main charge also be compacted?
>A.   Yes. To allow as much waves to blast the molecules apart simultaneously. 
>     Loose charge material cause micro delays, a factor which will 
>     drastically decrease the explosion in real time.

Forget the micro-delays. Ideal compaction depends on the explosive.
Kinepak shoots best at a maximum density of 1.16 g/cc. ANFO 
performance drops off at densities above perhaps 1.05 depending
on column diameter. Most military explosives perform best near
their crystal density. TNT's sensitivity depends on a lot ao things,
but it is very hard to initiate, as cast, at near maximum density
in small quantities, even with a large primer. However, the powdered
material shoots well with a cap even when fairly highly compacted.

>Q.   In a plastic explosive main charge, how hard should the material be 
>     compressed?
>A.   1 ton per square inch.

Pressed by hand like you would with play-dough. Thats why they make
it plastic. Hard, powdered explosives often are pressed at a few tens
of thousands of psi.

>Q.   If it is compressed harder, what will happen?
>A.   The detonator wave will not be able to split the molecules of this main 
>     charge in time, resulting in a failed explosion.

If an explosive is dead-pressed. it is more difficult for the
blasting cap to do PV work on the material. It is the PV work that
is converted to the heat which does the initiating. Remember that
energy is, inter alia, force acting over a distance, which is
equivalent to pressure acting through a volume.

>Q.   Should the main charge be sealed in some steel container to cause 
>     maximum damage?
>A.   No. Only if shrapnel is important, otherwise the charge will be capable 
>     of doing substantial damage in its "naked" form. Remember, an explosion 
>     is the simultaneous break up of molecules, each capable of emitting a 
>     tremendous force of waves. Waves cause the damage and not the metal 
>     encasing.

Forget the "waves." The terminology is too vague. It is pressure, 
resulting from heat that causes the damage. 

Jerry (Ico)

From: (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: blasting cap
Date: 26 Jan 1996 21:38:41 GMT

In article <4ebasr$>,
(Arthur Ulichny) says:

> Could someone please tell me the function and purpose of a blasting cap? I
>see so many postings about them and I have no idea of what they are.

The function of a blasting cap is to initiate a secondary 
(insensitive) explosive charge such as a block of plastique
or dynamite. The cap achieves this by exploding and generating
a very high shock pressure which compresses a small portion of
the explosive material so rapidly that the shock continues to 
propagate through the charge, being sustained by the rapid 
chemical reaction initiated by the original shock.

The cap itself is typically made of pencil stub-sized thin metal
shell packed successively with a small lightly packed secondary
explosive charge (typically < 1 gram) a layer of primary explosive
(i.e. one that detonates on being heated) and a layer of match
composition which can be readily ignited by an electric wire or
the spit from a piece of black powder safety fuse.

Fuse caps, now less common than a few decades ago, were open at
the match end so that they could be crimped onto a length of
safety fuse which burned at a rate of about 0.5 meters/min.
Electric caps have a couple of wires bridged by a fine filament
precrimped into the open end of the cap shell to make the 
assembly waterproof In practice the filament may be coated with
match material to assure contact for ignition.

The match-primary explosive-secondary explosive assembly is
called an "explosive train" and may include additional elements
such as HE at two distinct densities or a delay element of some
material with a dependable deflagration rate to allow the cap
to detonate at a precise time after electrical initiaton.
Such "delay" caps, ranging from 25 milliseconds to several 
second, allow the design of borehole delay patterns that can
significantly improve mining, quarrying and road-building blast 
yields while reducing ground vibration.

In addition to the mainstream caps described above there are 
also detonators which utilize only electrical energy in place
of the match/primary explosive ingredients. Other variations
include friction-activated units called "stab detonators" and
commercial caps which contain no primary explosive but are
initiated by the mild shock of low-energy shock tubing (Nonel).

Jerry (Ico)

From: (Gerald L. Hurst)
Newsgroups: alt.engr.explosives
Subject: Re: Question & answers on
Date: 20 Jan 1996 22:53:05 GMT

In article <>, (Sweden) says:

> GLH> From: (Gerald L. Hurst)
> >Q.   What powders are used in detonators?
> >A.   HMTD, RDX and PETN are common powders ideally suited for detonators.
> GLH> Make that PETN and RDX used as base charges, but initiated by a
> GLH> separate primary explosive. HMTD is a primary explosive and one which
> GLH> is too unstable for use in blasting caps.  
>For a few years there have been comersial blasting caps for sale
>manufactured by Nitro Nobel AB without any primary explosives at all.
>They consist of a delay element transfering heat from the Nonel cord
>to a PETN charge behind a steel reenforcing cap. The PETN undergoes
>selfpropagated detonation (DTD transfer) and initiates a RDX base
>charge which does not have a steel cover - just the usual Al cover.
>This makes, indeed, for a much safer blasting cap.

I have previously described my own design of a secondary-only
blasting cap which was made entirely of plastic and initiated
by the flash from a hollow tube propagating an O2/Al dust
detonation - analogous to Nonel. I strongly suspect that
your "heat transfer" is actually a modest shock transfer,
although there ARE initiators that truly rely on DTD by
simple. but intense direct heating. If the if there is no match
compound or other additive between the Nonel and the PETN, it 
is less likely that the Nonel could directly transfer enough 
pure non-shock thermal energy to initiate the PETN directly. 
Shock initiation is achievable by variable density and particle 
size control of the PETN.

If they are using steel plates in the caps, do they not worry 
about shrapnel hazards? 

Another interesting (very non-commercial) cap works exclusively
by chemical reaction by bringing two substances together
under the right conditions. Sorry I can't spell it out, but
it might not be too healthy for the teenybombers. We used
such a mechanism in prototype air-scatterable mines in Viet 
Nam days.

Jerry (Ico)

From: (Gerald L. Hurst)
Newsgroups: alt.engr.explosives
Subject: Blasting cap strength
Date: 27 Dec 1995 22:59:28 GMT
Organization: Consulting Chemist

Many thanks to Tip (Tom Peregrin) for including in his web
page ( a 
scanned copy of the BATF Orange Book of regulations. An 
undertaking like that requires a lot of time and patience.

My personal copy of the Orange Book is dated 1981, but I doubt 
that much has changed. However, Tip's electronic version allows
me to find things the bureaucracy did not see fit to put in
the index.

The blasting agent definition caught my eye immediately, because
it inadvertently defines BATF's concept of a number 8 test 
blasting cap. According to them, it is a cap containing 2 grams
of mercury fulminate/KClO3 mix (old formulation) or the 
"equivalent" modern base charge of 0.4 - 0.45 g of PETN at a 
density greater than 1.4 g/cc. We're talking about 6-6.8 grains 
of PETN in the old parlance.

The number 8 test blasting cap according to Mel Cook has exactly
8 grains of pressed (1.5 g/cc) PETN, and indeed his other test
caps contain base charges numerically equal in grains to their
series number.

The military mistrusts commercial blasting caps for initiating
plastic explosive and demolition charges, so they make a 
"Special" EBC that uses 13.5 grains of PETN or RDX. This cap
in now known as the "US Army Special Blasting Cap," and is
identical to what we used to call a "Special Army Engineers'
Blasting Cap," It is also informally referred to as a
"Military Number 8" because its base charge weight (13.5 
grains or 0.9 g) falls at the very top of the weight range 
defined by some authorities for number 8 caps (0.55-0.9 g).

As if the confusion were not enough already, the status of 
number six caps is even more vague. Cook says a number 6 has a
base charge of 6 grains which is precisely 3/4 of the charge
he assigns to a number 8 (8 grains or 0.55 g). However, on
the original fulminate/chlorate scale a number 8 cap was held
to be exactly twice as strong as a number 6. 

The British ASA detonators have variable weights of base 
charge depending on the explosive chosen. For PETN the
respective weights for number 6 and number eight base charges
are 0.22 and 0.45 g.  These numbers agree very well with the
idea stated above that a number 6 should be half as strong
as a number 8. What is more interesting is that the British
definition of "number 8" is closer to that of BATF than are
either the US military or commercial standards. I suppose
the definition may have been chosen as a convenience for
purposes of international trade.

There is some question in my mind as to whether commercial
blasting caps may have grown stronger. When dynamite was the
primary class A explosive, a number six cap was more than
adequate for all situations. The modern water-based explosives
are much less sensitive, so the number eight cap has pretty
well taken over. Not all number eights are equal. Seismic
caps such as du Pont's SSS are built to withstand very high
hydrostatic pressures. I always had the impression that such
caps might be a little on the strong side of typical number
eights, but I never actually tested their relative output.
Modern explosives would certainly invite the use of more
powerful caps within the generally accepted guidelines.

I can say that although the military mistrusts commercial
number eight caps for C-4 initiation, I had only one failure
over a period of years using SSS caps, and I could anticipate
that one from the handling of the explosive and the poor 
detonator contact.

Jerry (Ico)

From: (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: blasting caps.
Date: 4 Jul 1996 19:31:15 GMT

In article <4rgrnh$>, (murr rhame) says:

>Yates <> writes:
>>Does anyone know what a class 4 blasting cap is? A class 11 blasting cap 
>>is a blackpowder gun detonater, right?  Please explian the scale to me! 
>A number 4 blasting cap is blasting which has roughly the same explosive
>power of a old style blasting cap which contained 4 grains of mercury
>fulminate.  Modern blasting caps use different explosives. 
>A number 11 percussion cap is the most common size of cap used to fire
>caplock firearms.  Percussion caps are sized by their diameter so that
>they fit the gun's nipple correctly.  All percussion caps for black powder
>arms contain roughly the same amount of primer.  Black powder percussion
>caps are not blasting caps.  Black powder guns don't detonate. 

One might also mention that there is no such thing as a Number 4 
commercial blasting cap.  Caps below number 6 are only used for the 
limited purpose of establishing the minimum sensitivity of explosives
in the laboratory and there is no universal standard for the strength 
of such caps.

Murr's "strength = grains of Hg fulminate" is as good as any, but a 
modern #8 cap now contains up to 13 or so grains of PETN, a material 
much more potent than Hg fulminate.  In my youth a number 6 cap was 
said to hold 6 grains of base charge, not Hg fulminate though -I'm
not THAT old:)

Jerry (Ico)

From: (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Ignition by current
Date: 6 Oct 1996 20:00:20 GMT

In article <>, (Mr M.P.
Cresswell) says:

> Hi There,
> How can one be certain that devices set off by electrical charge ( to
>ensure proper safety by correct timing), are not liable to explode due
>to eddy currents generated by long copper wires ? ... how would one
>safely "earth" electrical detonators, and prevent static buildup ??

The lead wires are kept shunted until just before the connection is
made.  If one is working near power lines the leadwires are always kept
close to and parallel to the earth.  Air potential meters are available
to warn when voltage gradients begin to rise as prior to a lightning
strike. Ground currents near power lines can also be measured. In the 
event of sudden thunderstorms one gets the heck out of the area where 
charges are being wired up.  Conductive shoes or legstats are worn when 
possible. Radio broadcasting is prohibited in blasting areas (although
low wattage transmissions such as hand-held CB are unlikely to set off

Modern Staticmaster (R) type blasting caps are equipped with 
nonconductive junctions which become conductive when high voltage is
applied across the two leadwires.  The activated junction functions 
as a shunt and bypases most of any current which would otherwise have 
passed through the bridgewire (filament).

With static or other high voltage electricity one must be aware that
an arc from the leadwires to the metal cap shell can potentially
cause ignition even though the current does not pass through the 

With fuse caps one must be certain that the BP in the fuse is of the
non-conductive type as provided by reputable manufacturers.  Conductive
fuse can and has contributed to accidental explosions caused by 
static electricity flowing through the BP column in the fuse to the
match composition in the explosive train of the cap.

Jerry (Ico)

From: (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Electric Matches (SQUIBS)
Date: 8 Jan 1997 03:42:40 GMT

In article <5av0qu$>, (Bob
Pribusauskas) says:

>In article 
> says...
>>Does anyone happen to know the resistance of a SQUIB before firing?  I'm
>>trying to desingn a power supply to fire off multiple rounds at once.
>>any help would be appreciated.
>If you want the res of a squib you might try Alt. Eng. Explosives I'm sure 
>someone there would know the straight poop. Electric matches used in fire 
>works are usually figured at 1.5 ohms. The ones I have had metered out at 
>about 1.8 to 1.9 ohms.  Each brand of match will probably vary somewhat. 

In blasting caps the standard filament resistance is about 1.5 ohms.
The actual measured resistance of the whole unit is higher because
of the resistance of the legwires.  Legwires come in various lengths
and it is therefore left to the blaster to calculate the total 
resistance of his or her proposed circuit.  Blasters handbooks often
contain tables giving the resistance of cap assemblies with various 
legwire lengths.  Many squibs are merely incomplete blasting caps
without the HE element(s).  They may contain additional deflagrating
material to augment the electric match portion.

Jerry (Ico)

From: (Gerald L. Hurst)
Newsgroups: alt.engr.explosives
Subject: Re: Electronic detonators?
Date: 3 Jan 1998 18:46:14 GMT

Back in the 1960s and earlier, the #6 cap was the industry standard based
on common use with the highly sensitive nitrglycerin explosives common
at the time.  Number 8 caps were then widely used only in the seismic
industry.  The premier seismic cap was the duPont SSS which was
strong enough to initiate C-4 with reasonable certainty if one was careful
to set the cap in good contact.  Seismic caps had stronger shells and could
withstand hydrostatic pressures up to as much as 10,000 psi.  The SSS
cap had a dimpled end which functioned as a miniature shaped charge.
The dimple had no effect on initiating ability but probably increased the
 pressure resistance of the casing.

The number 8 cap became  more common in response to the need for
stronger initiation when using water-based explosives and cast primers
which usually contained too little sensitizing RDX or PETN in the blends
with TNT.  In those days the conventional wisdom was that military
detonators were about twice as strong as commercial #8s.  This was not
quite true because the #6 then typically contained about 6 grains of base
charge, slightly less than half that of the military "engineer specials."

As water gels and emulsions proliferated, #8s became stronger and for
all practical purposes equivalent to the military detonators.

The numbering system for caps using modern HEX was certainly somewhat
arbitrary and depended on who was making the cap and in what country.

"Standard" blasting caps were made in all numeric strengths up to at least
#12 for sensitivity testing but only #6 and #8 were available commercially
except by special order.  Of course, there was no single standard which
was universally accepted.

The adoption of stronger blasting caps by the industry came a little too late
for my convenience.  I spent a lot of time working to make Kinepak
formulations reliably function with #6s.  Fortunately, the cost of a #6 fuse
cap was only about 6 cents so we got a lot of bangs for our bucks. :)

Jerry (Ico)

In article <>,
Michael Fanning <> says:

>  Actually, the antiquated use of a number to describe the power of a
>blasting cap (also an increasing archaic useage, detonator is becoming
>preferred in the international community because it is used in several
>languages - just check out the IME warning statement on one the next time you
>see one - it probably doesn't say "BLASTING CAP" any more) harkens back to a
>day when mercury fulminate was used as the base charge.  The number referred
>to the equivalent power of a blasting cap for initiating explosives to a
>reference scale based on a mixture of MF and, I believe, potassium chlorate.
>In more recent years, the power standard was standardized at #6 and #8 as
>being the primary useful sizes and later #6 was dropped due to the increasing
>insensitivity of newer explosives such as emulsions and water gels.  #8 is
>used as the standard to determine blasting cap sensitivity of an explosive
>(which determines the shipping, storage and other restrictions required).
>Most blasting caps today are more powerful than a #8 and may be referred to
>as #8+ or #12.  On the original scale, however, these designations didn't
>exist and are simply extrapolations of "power" for reference by users.

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