Faraday to James Pattison Cockburn   20 June 1843

Royal Institution | 20 June 1843.

Sir

Having considered the paper¹ you placed in my hands, in conjunction with the objects and circumstances which were presented to us in our joint visit to the Waltham Abbey Gunpowder Works², I proceed to give such answers to the enquiries contained in the former as I am able, with my best judgement to make; begging you, however, to remember that it is with diffidence I speak, as one having only a theoretical acquaintance with these matters in comparison with that practical knowledge which is possessed and can be gained only by those who are occupied in the continual observance & practise of the works. With this degree of reservation I shall answer freely to the points in order[.]

With respect to the sieves used in granulating, <(1)>³ and the possibility that the different metals employed in them, conjoined with the water and nitre of the gunpowder could have produced by any galvanic effect the ignition of the powder in them it is my strong opinion that it could not: and that no inflammation of gunpowder has ever occurred at the mills from such a cause.

In reference to the incorporating mill it appears that they are usually either of marble or of iron; and further that when of marble, chips & fragments <(3)> have been known to separate and pass, on with the gunpowder, to the pressing or granulating houses. This it appears to me must be, as is said, a source of serious danger and in that respect iron appears to be superior. To this may be added that there never can be an absolute certainty that the limestone does not contain portions of grit, which if present are causes of increased danger. Iron is considered in the paper as a less ignitable substance than stone <(6)>, by which I conclude it is intended to say it is less liable to ignite the gunpowder, (it is more ignitable itself,) which is probably true under the circumstances; and it is assumed that in certain cases of ignition some grit <(4)> must have been present to cause the effect. I cannot however remove from my mind the impression that either between iron and iron or marble and marble the ignition of gunpowder is possible, as an extreme case, without grit;- for if a thin film of well incorporated powder, and a slight irregularity in the surface of the roller or bed, and the grinding twist of the roller, with its great weight should all, by any rare conjunction of circumstances, come into simultaneous effect at one spot, I can easily conceive the gunpowder might be fired without any grit. Of course this is generally avoided by the care taken to spread the composition uniformly <(7)>; I am only speaking here of a possibility[.]

It still seems to me that the roller might be iron and the bed of some hard wood, based upon either iron or stone <(8)>; and if this were practical it would relieve my mind from much of the impression of the risks of the incorporating houses. I do not see that, whilst there is powder between the runner and the bed, the wood would suffer much:- but I offer it only as a suggestion. How the wooden beds should be laid or bound together, and whether it should be with the grain upwards, or lengthways would be matters for practical consideration.

No galvanic effect <(5)> need be apprehended from the use of iron in the incorporating mills, nor from the metals in the boxes at the press house (10, 11).

In the press house the powder is described as being put into boxes <(9)>, and, that gunmetal plates are introduced with the powder. If these boxes are metal within, and the edges of the introduced metal plates are liable to come against the inside of the boxes, then it seems to be very possible, that when great pressure is on, the settlement of the mass under the continued or increased pressure might cause such friction as should at some unfortunate moment inflame the powder between the two surfaces; but I am not aware whether any precaution is taken to keep the plates from ever touching the sides; or whether the box is made of, or lined with wood.

I pass to the granulating machine; and apprehend no harm from its frame work being constructed of gunmetal <(12)> and the axis of iron <(13)> by any galvanic effect which they can produce <(16)>. Further if the cogs of the wheels, being well formed, are kept, not merely well oiled <(14)> but, wet with oil by a process of lubrication as independant as possible of the care of the men, like that figured for instance, I think they would be as safe as, or safer than, bands or any other means of conveying the power. Where the wheels are large enough to allow of the use of wooden cogs, my impression is in their favour.

In the granulating rollers the intent is, and the arrangement such, that the teeth shall never touch each other, and that nothing but powder shall pass between them <(15)>; and these conditions being fulfilled, there is as little danger as possible in the process. I do not know whether the opportunity has yet occurred of observing whether the temperature of these rollers rises in the process, by the friction of their teeth against the pressed cake. It is a point worthy of attention and remark in the first instance[.]

In reference to the question of Zinc roofs <(17)> I at first considered them as unexceptionable⁴, and so they are as far as they could act by galvanic action. But there is an effect which they in common with copper, lead, or any metal roof <(2)>, may by possibility produce of the following nature which might be dangerous. If we suppose a roof of metal plates and a thunder cloud or electric atmosphere over head, a discharge of lightning, though at some distance, might cause sparks of electricity to pass between the plates, or between them & the metal spouts &c, because of their being under inductive action at the time. The sparks might be very small; but yet fine gunpowder dust might be in the place where they occurred, and though it is perhaps ten thousand to one that they would fire it, still the possibility remains. It⁵ is true, that, if all these pieces of metal were in metallic communication with each other and with a good lightning rod, the sparks would not occur; but even if put in such communication at first, the action of sun, air, heat, cold &c. might gradually cause separation and then the circumstances would be as above. Besides this, it is not the mere fixing of two sheets together or nailing them over each other that makes electrical contact; for the oxide⁶ upon their surface is quite sufficient to keep them so far apart as to be electrically separate[.]

With reference to the charcoal I am not able to suggest any thing definite[.] Experience is the only safe guide[.]

Again begging you to remember that I answer with diffidence on all the practical points⁷[.]

I have the honor to be | Sir | Your faithful Obedient Servant | M. Faraday

Colonel Cockburn R.A | Director of the Royal Laboratory | &c &c &c &c | Woolwich

Endorsed on the copy in PRO SUPP5 / 267, f.105: It seems to me that a safer roof cannot be constructed than that proposed (of galvanised iron) not that I mean to set my opinion in opposition to Mr Farraday, but I do not think that it would be possible to construct a roof that would not be liable to the similar objections if it fulfilled all the conditions required. Mr Farraday says that if all these contingencies should occur it is still 10,000 to 1 against the powder igniting. 2y Are there not many other combinations that might occur in the varied process of making gunpowder which would be less than 10,000 to 1? GH⁸