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Tuesday, September 05, 2006

Sulfates, Barium Sulfate.

Most sulfates are not water soluble, are geologically stableand can be easily and cheaply obtained by mining, ratherthan having to be produced through complicated and expensivechemical processing. Therefore sulfates pass the first testfor possible inclusion in any pyro formula; they areinexpensive. Indeed native sulfates such as barite (BaSO4)and celestite (SrSO4) are the starting materials for otherbarium and strontium compounds used in fireworks.Sulfates certainly appear attractive because their oxygencontent compares favorably with that of metal chlorates,perchlorates and nitrates, as Table 1 illustrates. Also acomparison of the heat evolved from reaction of aluminum andvarious oxidizing agents again shows that sulfates comparefavorably with more familiar pyrotechnic oxidizers. (SeeTable 2.)

Table 1
Percent oxygen contained (percent by weight) for variouspyrotechnic oxidizers and sulfates, for the anhydrouscompound.
Nitrate Chlorate Perchlorate Sulfate
Ammonium 0.60 0.47 0.54 0.48
Barium 0.37 0.32 0.38 0.27
Calcium 0.58 0.46 0.41 0.47
Copper 0.51 0.42 0.49 0.40
Gadolinium 0.42 0.35 0.42 0.32
Lithium 0.69 0.53 0.60 0.58
Magnesium 0.65 0.50 0.57 0.53
Potassium 0.47 0.39 0.46 0.37
Sodium 0.56 0.45 0.52 0.45S
trontium 0.45 0.38 0.45 0.47

Table 2Heat produced (cal/g) from a mixture of an oxidizer orsulfate with aluminum. Values from AMCP 706-185(1967) and/orVasilev (1973) (*).
Sodium perchlorate 2,600
Lead nitrate 1,500
Sodium chlorate 2,500
Barium nitrate 1,400
Potassium perchlorate 2,400
Cu sulfate 1,400/1,560*
Potassium chlorate 2,200Ca sulfate 1,300/1,470*
Sodium nitrate 1,800Na sulfate 1,200/1,360*
Potassium nitrate 1,800K sulfate 1,200/1,180*
Lithium sulfate 1,620*Barium sulfate 900/910*
Magnesium sulfate 1,610*
Lead sulfate 800
Ammonium nitrate 1,600

However, low cost is not the only criteria for selectingoxidizers for use in fireworks compositions. A quick checkof Table 1 reveals several oxidizers with high oxygencontent, for instance, calcium, sodium, and ammoniumnitrates, sodium chlorate, and magnesium perchlorate.However, of these only sodium nitrate has found use, albeitlimited primarily to military pyrotechnics. All of thesecompounds are hygroscopic and therefore unusable in the realworld. In fact, magnesium perchlorate is used as a dryingagent under the trade name of "Anhydrone".There can be no doubt that the largest problem concerningthe use of sulfates as oxidizing agents is their waters ofhydration, for example:Na2SO4-10H2O and CuSO4-5H2O. Although the ten extra oxygenatoms in sodium sulfate raise its total oxygen content from45% to 70%, this extra oxygen contained in the waters ofhydration is not available for productive work. In truth itonly gets in the way, since a large amount of heat isrequired to first remove the water of hydration from acomposition's outer surface before the ignition temperaturecan be reached. Then once the reaction becomes selfsustaining, even more heat, produced by a burning star forinstance, will be removed from the reaction in the form ofvaporized water. (It should be noted that the latent heat ofvaporization for water is 540 calories per gram of water at100° C. This value represents heat that must be supplied bythe pyrotechnic reaction to change water at 100° C intosteam at 100° C.) There is also the possibility, inmagnesium containing compounds, of the water vapor reactingwith the magnesium forming hydrogen and magnesium oxide,effectively removing a large amount of fuel, with littlegain in heat. In the case of sodium sulfate decahydrate,where 56% of each molecule is water, 31,920 calories of heatwould have to be supplied simply to remove all the water ofhydration in the form of steam from each 100 grams ofsulfate. For example, in a composition using potassiumperchlorate as the oxidizer and aluminum as the fuel, 13.3grams of aluminum and potassium perchlorate would be neededjust to remove the water from each 100 grams of sodiumsulfate decahydrate, before any useful work (heat and/orlight) would be produced!As a further complication, the temperature at which watersof hydration are liberated varies from sulfate to sulfate,e.g., sodium sulfate decahydrate loses all its water at 100°C while manganese sulfate monohydrate does not lose all itswater until the temperature reaches 400-450° C! And toreally complicate things, manganese(II)sulfate can exist aseither mono, tri-, tetra, penta, hexa, or heptahydrate!Although the tetrahydrate is the most common form.However, US Patent 2,885,277 claims to make use of thewaters of hydration in magnesium sulfate heptahydrate,MgSO4-7H2O (Epsom salts), to produce hydrogen gas when thesulfate is reacted with magnesium. It is further claimedthat this combination will function as either a torch or asalute. It would be well to note that Ellern (1968, p. 272)expresses doubt concerning the safety and utility of suchmixtures.The use of sulfates as oxidizers suffers from yet anotherproblem. As Dr. Conkling (in press) has pointed out "Inpyrotechnics, the solid liquid transition appears to be ofconsiderable importance in initiating a self propagatingreaction. The oxidizing agent is frequently the keycomponent in such mixtures, and a ranking of commonoxidizers by increasing melting point bears a strikingresemblance to the reactivity sequence for these materials."Unfortunately the melting point of most sulfates is muchhigher than either chlorates, perchlorates or nitrates. Onlyfour sulfates (manganese, copper, zirconium and iron) havemelting points below that of barium nitrate, and these fourare well hydrated (tetra or penta). Melting points aresummarized in Table 3.

Table 3Melting point for various anhydrous oxidizers and sulfates.Values are from the CRC Handbook. d decomposes, sd slightdecomposition.

Copper perchlorate 82
Ag perchlorate 486
Iron perchlorate >100d
Thorium nitrate 500
Strontium chlorate 120
dTh perchlorate 501
Lithium chlorate 128
Ba perchlorate 505
Scandium nitrate 150
Sr nitrate 570
Manganese(III) sulfate 160
dBa nitrate 592
dAmericium nitrate 170
Zn sulfate 600
Copper sulfate 200
sd 650
dTh(I) sulfate 632
Silver chlorate 230
Silver sulfate 652
Lead chlorate 230
Mn(II) sulfate 700
Lithium perchlorate 236
Lithium sulfate 845
Sodium chlorate 248
Nickel sulfate 848
Magnesium perchlorate 251d
Sodium sulfate 884
Lithium nitrate 264
Ytterbium(III) sulfate 900
Calcium perchlorate 270
Yttrium sulfate 1000
Sodium nitrate 307
Cesium sulfate 1010d
Rubidium nitrate 310
Rubidium sulfate 1060d
Potassium nitrate 334d
Potassium sulfate 1069
Calcium chlorate 340
Samarium sulfate (basic) 1100
Potassium chlorate 356
Magnesium sulfate 1124d
Potassium perchlorate 400d
Lanthanum sulfate 1150
Zirconium sulfate 410
dsulfate 1170d
Cesium nitrate 414
Calcium sulfate 1450
Barium chlorate 414
Barium sulfate 1480
Iron sulfate 480d
Sr sulfate 1605d
Sodium perchlorate 482

It is evident that getting compositions based on sulfates asoxidizers to ignite while not impossible ... is not going tobe easy. There can be no doubt that it is going to take anextremely hot ignition source!Copper sulfate with its low melting point looks like a primecandidate but again, the water of hydration is a problem.Exposed to moist air, CuSO4 becomes CuSO4-H2O, and whenwetted, CuSO4-5H2O. Also, because copper sulfate is watersoluble, it is seldom found in native form (chalcanthite).Therefore it is manufactured from copper metal and sulfuricacid, and as a result fails the first test, it is not cheap.It is also not safe with chlorates.Although certainly attracting because of their low costoxygen content, sulfates have for the most part, not beenemployed as oxidizing agents. However, them have found theirniche in strobe formulas.Vander Horck (1974) reported on several formulas usingcalcium and copper sulfates demonstrated to him by BobWinokur who later (Winokur, 1974) made additional commentsabout them. Further Dr. Shimizu (1981) presents severalstrobe ("twinkler") formulas using sulfates, i.e.,strontium, barium, sodium and calcium. Advantage is taken ofthe great difficulty of igniting and then sustainingignition in sulfate based compositions. Therefore flashes oflight are produced each time the sulfate reaches its meltingpoint or decomposition temperature, burning commences andshortly thereafter extinguishes only to repeat, producingthe strobe light effect.Sulfates have long been used in color flame compositionsmore for their metal than oxygen content. However, for themost part, the color produced by sulfate based compositionsnot containing metal fuels such as aluminum or magnesium,will be found to be less than satisfactory, since only metalfuels are capable of producing the high temperaturesnecessary to melt or decompose most sulfates. The use ofvarious sulfates is detailed below:Copper sulfate: In older literature, e.g. Kentish (1878)compositions for blue flames can be found using coppersulfate and potassium chlorate, where the copper ion is usedto produce the blue color. THIS COMBINATION IS DANGEROUS.Safer and more effective blue formulations are available.Barium sulfate: Troy Fish (1981) recommends the use ofbarium sulfate in parlon bound green stars. He notes that asa result of barium sulfate's extreme insolubility (0.000413grams per 100 ml of boiling water!), it is one of the fewnontoxic barium compounds. I have been able to locate onlyseven formulas using barium sulfate, and all seven useeither magnesium, aluminum or magnalium.Calcium sulfate: Despite the many obstacles noted above,calcium sulfate hemihydrate (plaster of Paris) [CaSO4-1/2H2O] has been used as an oxidizer in fireworks andpyrotechnics: In combination with sodium and barium nitratein white light compositions (Ellern, 1968, formulas 36, 37and 38), as an incendiary when combined with aluminum (USPatent 2,424,937, Vol. 3 of the "Black Book", 1982), oraluminum and magnesium sulfate (US Patent 4,381,207), andwhen compounded with aluminum, Teflon, and sulfur (US Patent4,349,396) as a metal cutting torch.Calcium sulfate combined with either aluminum or magnesiumhas been suggested as a "flash report" mixture! (Sanford,1974)This sulfate is found in pink tableau fire or starcompositions using potassium perchlorate as the oxidizingagent. Weingart (1947) has the only modern formula I have been able to locate that uses calcium sulfatewithout either aluminum, magnesium or magnalium.Potassium sulfate: The Technico Chemical Receipt Book 1896long ago recommended the use of potassium sulfate in bluecompositions. There is only one modern formula usingpotassium sulfate, Dr. Shimizu's white "twinkler" usingmagnalium as the metal fuel.Strontium sulfate: This sulfate had long ago been used inthe production of red or purple flames. However, there areno formulas using strontium sulfate in Lancaster, Ellern orWeingart. There are however, three "twinkler" formulas inShimizu using strontium sulfate. All three containmagnalium.Sodium sulfate: I have been able to locate only fourformulas using sodium sulfate, all by Dr. Shimizu, who usessodium sulfate in combination with magnalium for yellowstrobe stars.Manganese sulfate: Perhaps the most interesting use ofsulfate is the addition of manganese sulfate (MnSO4 H2O) toaluminum sodium nitrate flare compositions. Farnell etal.(1972) discovered that this compound alters "thedecomposition of sodium nitrate to form oxides of nitrogenrather than its normal decomposition products of nitrogenand oxygen." This change results in a 55% decrease inburning rate, a 155% increase in luminous output, and a 466%increase in luminous efficiency!Although not a mainstays of the fireworks trade, sulfateshave found employment along with the proverbial kitchensink, used frying pans, oil of spike and philosopher's wool!!!

Literature cited
AMCP 706185, 1967, Engineering Design Handbook, MilitaryPyrotechnics SeriesPart 1; Theory and Application. NTIS AD 817071.Black Book, 1982, Improvised Munitions Black Book, Vol. 3.Desert Publications.Conkling, J., (in press), The Chemistry of Pyrotechnics andExplosives: Basic Principles and Theory. Marcel Dekker, NewYork.CRC Handbook of Chemistry and Physics, 1981, 62nd edition.Ellern, H., 1968, Military and Civilian Pyrotechnics.Chemical Publishing Inc., NY.Fish, T., 1981, Green and other colored flame metal fuelcompositions using parlor. Pyrotechnica Vll, pp. 2537.Farnell, Westerdahl and Taylor, 1972, The Influence ofTransition Metal Compounds on the AluminumSodium Nitrate Reaction. Third International PyrotechnicsSeminar.Kentish, T., 1887, The Pyrotechnists Treasury, The CompleteArt of FireMaking. Chatto and Windus, London.Sanford, R., 1974, Plaster of Paris flash powders, AmericanPyrotechnist Fireworks News, p. 527.The Technico Chemical Receipt Book 1896.Merck Index, 1983, The Merck Index: An Encyclopedia ofChemicals, Drugs, and Biologicals. Merck and Co., 10thedition.Shimizu, T., 1981, Fireworks: The Art, Science, andTechnique. Maruzen Publishing Co.US Patent 2,424,937, July 1947, Incendiary Composition.US Patent 2,885,277, May 1959, Hydrogen Gas GeneratingPropellant Compositions.US Patent 4,349,396, September 1982, MetalCutting Pyrotechnic Composition.US Patent 4,381,207, April 1983, Pyrotechnic Composition.Valsilev, A.A., et al., 1973, Combustion of mixtures ofmetal sulfates with magnesium or aluminum. Translated fromRussian. NTIS AD 785988, 5 pp.Vander Horck, M.P., 1974, Unconventional star compositionsdemonstrated. American Pyrotechnist Fireworks News, 7(4),issue no. 76, p. 506.Weingart, G. W., 1947, Pyrotechnics. Chemical PublishingCo., NY, pages 61 and 134.Winokur, R., 1974, More on unconventional stars. AmericanPyrotechnist Fireworks News, 7(5), issue no. 77, p. 516.

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