While more zinc than iron is required (the weight ratio of metal to acid must be 0.66:1 rather than 0.57:1), and in the seventeenth century zinc would be the more expensive metal. In the early-twentieth century the byproduct zinc sulfate was of greater commercial value than iron sulfate. (Teel 42). Iron sulfate may be used in manufacturing other iron compounds and iron gall ink, as a mordant, and as a developer in the collodion process. Zinc sulfate may be used to remedy zinc deficiency in soil, to coagulate viscose rayon into fibers, in the manufacture of lithopone, in zinc plating, and as a mordant, preservative, and corrosion inhibitor.
Also, zinc is higher on the reactivity series, and hence hydrogen production is likely to be faster. (And still more reactive metals, such as aluminum, will react even with water, and the acid can be dispensed with-see below.)
Still, the iron-acid reaction appears to have been respectably rapid in practice. On April 5, 1862, Lowe arrived at the lines shortly after noon and ascended at 5:20 (Crouch 375). So, in five hours, he inflated a balloon, and the smallest of his balloons was 15,000 cubic feet. (358).
As the reaction progresses, the metal is covered with the sulfate, protecting it from further reaction. In the nineteenth century, mechanical and hydraulic devices were devised to scrape or wash off this coating. (Molinari 133).
The vitriol process was first used for ballooning by Jacques Charles in 1783; he reacted 1100 pounds of iron filings with 550 pounds of acid. It took three days and nights to produce enough hydrogen to fill a balloon with a capacity of less than 1400 cubic feet. (Sander). Note that Charles used too much iron and not enough acid, a mistake that someone with modern high school chemistry wouldn't make. It's possible that he had problems exposing all of the iron to the acid.
In 1785, Aime Argand produced hydrogen for the Blanchard-Jeffries crossing of the English Channel. Jeffries paid 100 guineas for materials, most of which was "spent on the most expensive item, the acid." Argand's work area was about 100 feet in diameter. He placed fifty pounds of "parings of iron plates" and one hundred pounds of "cast iron trimmings" in the bottom of each of twenty-six 54-gallon half-barrels, and added hundred pounds of sulfuric acid to each vessel. That implies use of 3,900 pounds of iron and 2,600 pounds of sulfuric acid to produce. the required 9,000 cubic feet of hydrogen. The half-barrels were capped with an upended tub with a central pipe; hydrogen rose through this pipe into a leather hose, which conveyed it to a larger barrel for "purification and cooling before transfer to the balloon." Occasionally, the half-barrel was opened and the iron stirred around with an iron rod, to expose fresh surface. (Crouch 81).
The acid process was also used by Union balloonists during the American Civil War. Thaddeus Lowe designed the army's portable gas generators. The generator was a strongly braced wooden box 11 feet long, 5 feet high, and 3 feet wide, which meant that it was able to fit in a standard wagon body. There was a manhole on top and a rear door. It was acid-proofed inside, with shelves to hold 3,300 pounds of iron filings, submerged in three feet of water. Sulfuric acid was introduced through a rooftop copper funnel, and the produced gas rose through a six-inch hose. This hose was connected to the cooler box, which was five feet long and has a smaller box inverted inside. The cooler box was partially filled with water; the gas entered the cooler box underwater and bubbled up, around cooling baffles, eventually escaping into a second hose. This conducted the gas to the purifier box, which was similarly constructed, but contained a limewater solution. (That would absorb carbon dioxide.) A third, twelve-inch hose ran from the purifier box to the balloon. (Crouch, 358; Tunis 88-89).
A British observer reported in 1862 that the iron was inexpensive since "any old iron" would do, and that the sulfuric acid "in large quantities is cheap, and with proper precautions, very easy to carry." (Templer 174).
The acid-iron process was used to fill the giant (118 feet diameter, 882,900 cubic feet) captive balloon erected by Giffard for the 1878 Paris Exhibition; 190 tons acid and 80 tons iron were consumed. (Baden-Powell 741).
In 1883, if 250 pounds of iron were used to make 1,000 cubic feet of gas, the cost of production was 1? 5s per 1,000 cubic feet. A good portable apparatus filled a 6,000 cubic foot balloon in four hours. (Powell).
In the Andree North Pole expedition of 1893, zinc was used; the costs in kroner were estimated to be 1950 for the apparatus (producing 5300 cubic feet/hour), 3,000 for the raw materials (zinc and sulfuric acid, in 20% excess), and 1600 for the technical expert's salary. However, the expert decided to use wrought iron instead of zinc. (Capelloti 149). Nonetheless, the 1896 British manual of military ballooning favored zinc. (Taylor 169). In the 1632 universe, zinc is a rather rare commodity, and so our aeronauts will almost certainly use iron.
The acid process was again used by the 1907 Wellman expedition. My source mentions a number of interesting points, including that perfume is added to the hydrogen stream (by passage through sponges filled with muronine) so leaks are readily detectable, and the gas is piped through coke (to dry it), caustic soda (to remove residual acid), potassium permanganate (to remove arsine, stibine and phosphine?) and calcium of lime (to remove carbon dioxide?). (Capelloti 152ff).
Nonetheless, even in the early-twentieth century, the acid process was considered too expensive for large-scale industrial production, unless the hydrogen was simply a byproduct of producing a salable metallic salt. (Ellis 515). For example, in 1904, figuring zinc at 9.75 cents/kg and sulfuric acid at 1.75, 1 kg zinc and 2 kg acid would theoretically produce 32 grams (about 360 liters) hydrogen for 12.25 cents. Taking into account that the metal-acid reaction is usually incomplete, the electrolytic method (see Large Scale Production below) could produce the same volume with 800 ampere-hours (2 kwh @ 2.5V), with a then cost of power of 0.5 cents (hydroelectric) or 2.5 cents (coal/steam)(Englehardt 125). Ellis (518) adds that "the operating cost of an electrolytic plant [in 1917] is one-fifth that of a zinc-acid plant and there are no acid-eaten hydrogen pipes or freeze-ups in winter."
Base-metal process. This is alluded to by EB11/Hydrogen, which recommends reacting sodium or potassium hydroxide with zinc or aluminum, or zinc with an ammonium salt other than nitrate. The zinc-sodium hydroxide reaction produces hydrogen of high purity, but it has arsine (from the zinc) and caustic soda impurities.
Teel (44) says that the reaction of zinc with magnesium hydroxide has been used for ballooning.
In the Russo-Japanese War, the Russians reacted 30% caustic soda with aluminum scrap. They transported twenty-four generators and six coolers (the reaction generated a lot of heat) with the aid of fifteen horses, and this setup was sufficient to fill a 400 cubic meter balloon in thirty minutes. (Molinari 134).
Base-Carbon. A base like caustic soda may be reacted with coal to produce hydrogen:
4NaOH + C -› Na2CO3 + Na2O + 2H2.
The coal may generate methane, arsine or hydrogen sulfide impurities. (Teel 60).
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Alkali (Alkaline) Metal-Water. The most common reaction is
2Na (46 g) +H2O (18 g)-›2NaOH (62 g) +H2 (2g).
The reaction of water with sodium is much more vigorous than that with iron; the water need not be provided in the form of steam. In fact, the reaction had to be slowed down, for example, by supplying the water as a fine spray, or incorporating the sodium into a briquette with an inert binder. (Taylor 127).
Since water would be available in the field, only the sodium, a light metal, had to be transported. The catch was that metallic sodium was expensive-5s/pound in 1883, so 1,000 cubic feet of hydrogen would cost 22?. (Powell). Also, sodium was dangerous to transport, because of its reactivity with water.