An Investigation of the Influence of Charcoal on BP

Transcription

1 An Investigation of the Influence of Charcoal on BP Richard Harrison, 30 April 2014 Introduction Regarding the suitability of different charcoals for making black powder, there seem to be as many opinions as there are authors. The broad consensus seems to be that willow, alder and alder buckthorn (often called dogwood) are the best woods to use but it is by no means clear to me, at least by which criteria they are deemed to be the best. These three woods were all used at the Waltham Abbey Gunpowder Mills and, from reading 19 th century accounts, I get the impression that the chief concern at that time was consistency rather than raw speed or power. The fact that these woods could be grown locally and were therefore economically viable to use must also have been a contributing factor. Twentieth century research, mainly by military establishments, appears also to have concentrated on consistency, with an emphasis on accurate control of the carbonisation process and optimisation of volatile content. Apart from not particularly prescribing to the theory that a charcoal s performance is improved by the retention of volatiles, I don t have access to temperature-controlled ovens running in an inert gas environment, so I don t find such research to be particularly helpful. One other thread of work in this area is the results of golf ball tests by amateur enthusiasts. Unfortunately, most published results come from poorly defined and/or ill-considered black powdermaking techniques and don t seem to get much further than claiming that charcoal X is better than charcoal Y. Charcoal, being a natural product, must come with an almost infinite variety of properties. Even if made from a particular species of wood, there must be variations caused by differences of soil and climate and even the time of year at which the wood is collected (Waltham Abbey preferred spring-cut wood because it made the bark easier to remove). My approach, therefore, has been to reduce the emphasis on this or that species of timber and concentrate instead on simple properties, such as density and ash content, that can be easily measured for each charcoal, regardless of its origin. I started these trials with the intention of finding out what I could determine about the effect of different charcoals, using only equipment and resources that are easily and relatively cheaply available. As far as possible, I also wanted to avoid making any assumptions about factors that I had no means of determining, such as the volatile content and the microscopic or molecular structure of charcoal, or the precise nature of how black powder ingredients become incorporated during the milling process. To reduce the effort to a reasonable minimum, I decided from the outset to restrict my investigations to the behaviour of a single 75 : 15 : 10 formula, and to use a standardised, but

2 thorough, milling technique. The intention in the following trials, which I hope has been achieved, is that the only variable is the charcoal. The Charcoals The trials described here were made on twenty one different charcoal samples, some of which were made by me but the majority were supplied by others. They are made from a wide variety of types of wood. The range is: Acacia (genus Albizia) Alder Balsa Buddleia Jelutong Knotweed Leylandii Mixed woods (2 examples) Paulownia Pine (3 examples) Recooked BBQ (3 examples) Vine Willow (4 examples) As far as I am aware, all charcoals were made by the retort method and were carbonised at a temperature between about 400 and 600 degrees C. The ones I made were cooked at dull red heat until no further gases escaped. The possibly significant points are that I don t believe that any were prepared under rigid temperature control and that they all seem to have been prepared in such a way that they contain very little in the way of readily volatile material. Most of the charcoals were supplied in a reasonably finely milled form, although one or two contained larger pieces, up to a few mm in size. The charcoals were used as supplied, without any additional pre-milling. The Charcoal Properties I made the following basic measurements of each charcoal s properties: Ash content A weighed sample of about 3 grams was heated in a weighed small (40 ml) crucible until no visible unburned charcoal remained and the weight showed no further loss. The ash content was calculated as a percentage of the original sample weight, with an estimated accuracy of about ±0.1 on values that ranged from around 1.2% to 18.2%. Water content A separate weighed sample of about 3 grams was heated to 80 degrees for a period of one hour and then immediately reweighed. A number of trials indicated that there was no further weight loss if the sample was heated for more than one hour. The samples rapidly regained weight on returning to

3 room temperature and reached a value close to the original weight within 24 hours. The water content was calculated as a percentage of the original sample weight. The results varied from around 1.5% to 9.9% with an estimated accuracy of about ±0.2%. It is unlikely that this technique removed all adsorbed water and, in any case, the moisture content will vary from day to day, depending on the ambient temperature and atmospheric conditions. It is, however, a quick and easy method and was considered to be a sufficiently accurate indicator for the purposes of this investigation. Apparent Density Again, a simple and straightforward measurement was adopted. Each sample was taken in a 5 ml scoop, levelled by sliding a straight edge across the top, with no effort made to compact the contents or to take into account the differing degree to which the various samples had been milled. The content of four such scoops (i.e. 20 ml) was weighed and the density calculated. Recording the weight of each separate scoop allowed some estimation of the consistency of the measurement, which was generally within ±0.01 on values ranging from 0.09 to 0.41 g/cc. The question as to whether such a simple measurement gives a physically meaningful result was left to be decided by the results. Milling and Sampling The milling process broadly followed the method that I have described elsewhere ( No dextrin was added to the mixture, but otherwise the composition and methods were identical to those described in that document. Milling was, in all but one case, continued for a total of 8 hours and samples were taken at milling times of 30, 60, 90, 120, 150, 180, 210, 240, 300, 360, 420 and 480 minutes. For one charcoal, milling was exceptionally slow and in that case the milling time was extended by 3 hours, with further samples taken at 540 and 660 minutes. Burn Rate Measurements For each charcoal, the twelve samples were rammed into previously labelled and weighed ¼ inch ID by 5 cm long convolute cardboard tubes. The length of each rammed column was measured and each tube was reweighed to determine the mass of powder it contained. Each tube was wrapped in a paper nosing. A small charge of fine grain BP was placed at each end, to help determining the precise start and end of the burn. Ignition was by means of a short length of black match, as shown in the diagram. Each test burn was videoed and the resultant video converted to an audio file in order to measure the burn time as accurately as possible. Graphs of the burn rate, both in mm/s and g/s, for each sample were plotted, giving results of the form shown in the accompanying graphs.

4 In all cases the graph reaches a plateau in the performance so that, after a certain point, there is essentially no change in the burn rate with additional milling. This plateau region may be truly flat but in some cases it shows a very gradual change which may be a slow rise, or even a slow fall in the burn rate. This behaviour is seen to be more interesting when the data is plotted with a logarithmic time scale. All curves then appear to be a straight line up to some well-defined critical point, beyond which the line is effectively flat. This behaviour is clearer in some cases than in others (for example, when milling progressed quickly) but I have yet to find a case which is in disagreement with such an interpretation. As well as determining the peak burn rate it is, therefore, possible in all cases to find a reasonably precise measure of the characteristic rate of increase, plus an accurately measured time at which the improvement in burn rate ends. Results Dependence on Density One finding of this study is that there is a highly significant correlation between the maximum burn rate (both linear and in terms of mass per second) and a charcoal s apparent density, as shown in the following graphs. The relationship appears to be linear. The standard deviation from the best fit straight line for the mass burn rate is 0.04 g/s and that for the linear burn rate is 1.1 mm/s. Putting that another way, a simple measurement of the apparent density of a charcoal appears to predict its burn rate with an expected error of around ±10%. I have heard more than one person say that fluffier charcoals make the best BP but, given the wide variation in properties of such a natural product, I certainly did not expect such a striking result.

5 Dependence on Ash Content Again there is a strong correlation, for both measures of burn rate. In both cases the best fit curve is quadratic, with a maximum close to 7% ash content (6.8% for the linear rate and 7.1% for the mass rate). The standard deviations from the best fit curve, at around 10% of the measured values, are very similar to those for the burn rate-density relation: 1.1 mm/s for the linear rate and 0.05 g/s for the mass burn rate. Discussion The general form of the relationship between burn rate and ash content is the one result that I was expecting to see at the outset of this investigation. Assuming that the 75:15:10 formula is a stoichiometric mix for some particular charcoal (with around 7% ash content), it follows that charcoals with different ash content will contain either too little or too much combustible material for all the ingredients to consumed in the reaction. Energy will be used in heating the unconsumed substances, resulting in a slowing of the reaction. In an attempt to find more information about such processes, I examined the relationship between the burn rate and the sum of the ash and water content, on the grounds that this sum represents the total amount of non-combustible material. As expected, the graphs show the same general form, with maxima of the best fit quadratic curve at non-combustible content at around 11% (linear rate at 10.6% and mass rate at 12.1%). The graphs show rather more scatter than those for ash content alone which is, perhaps, understandable given that the water content is a less well defined and more variable quantity. I don t have any clear idea of the reason behind the relationship between burn rates and charcoal apparent density. I could speculate that the less dense charcoals have a more open structure and therefore lend themselves to more intimate incorporation, but I have no means of proving whether or not that is the case. Even if it were, there is no explanation of why the relationship should appear to be so highly linear.

6 Supporting evidence might come from observing a relationship between a charcoal s properties and milling time (for example, that the more dense charcoals took longer to reach the performance plateau) but the data show no such trend. A plot of the time to reach peak performance against apparent charcoal density appears to show random scatter, and plots of the rate of rise in performance parameters against density don t appear to be much more organised. I ll close this section with one or two words of caution. Firstly, I don t claim that my burn rate results are necessarily the best that can be obtained from each charcoal. My milling procedures were determined by the mill and milling media that I had on hand and it is quite possible that better (or worse) speeds could be achieved with different milling techniques. I would, however, expect the general trend of relative performances to be reproducible by any process that ensures a plateau of performance is reached for each charcoal. I have deliberately not identified the types of charcoal in my results, as I don t believe that any statement of the form Charcoal X is best for making BP has any great general validity. The evidence from my measurements is that different samples of charcoals made from a given type of wood can vary significantly and these differences can outweigh the effect of selecting one type of wood or another. I can t say whether the differences result from the growing environment, the precise details of the carbonisation process or any other factor. All I can say is that, once a charcoal has been made (provided it is carbonised at a sufficiently high temperature to eliminate the bulk of volatile substances 1 ) its burning behaviour in BP seems to be predictable with some degree of certainty from its physical properties. The Future The present tests relate to a relatively small number of charcoal types, many of which are traditionally associated with pyrotechnics. Although the list contains a few samples made from unusual woods, it would be useful to extend the range of charcoal types as widely as possible to see if the findings remain valid. A significant limitation of the results described in this document is that they only refer to the burn speed of a ¼ inch diameter column; there is no data relating to other qualities, such as gas production or the effectiveness as a propellant. BP behaves differently under different physical conditions and it would be useful to extend the experiment to measure the behaviour under a greater or lesser degree of confinement to see if the same, or other, relationships hold. I am convinced that there is meaningful information to be extracted from the time-dependent milling data, but I haven t found it yet. I can foresee that when (or if) I do, it may lead to further studies of ball mill behaviour. Acknowledgements I am grateful to Pyro-Gear and Niall, who not only supplied me with many of the charcoals used in the tests, but also provided valuable suggestions and advice. 1 I have some evidence that suggests an undercooked charcoal will produce a BP that burns more slowly than my data would predict. Others have also reported that heating charcoal to very high temperatures (say, 1000 degrees C) reduces its reactivity and hence its burn speed in BP.