Your best bet is to contact the Contra Costa County Historical Society for information on names of workers killed by explosions at the Hercules Plant. I doubt Alfred worked at the California Powder works in Santa Cruz County. This is from the "The Center of Land Use Information" Website information on Hercules Explosive Plant Site: "In 1893, California Powder Works moved its black powder production from Santa Cruz to this then remote area, up the shore from the new Giant Powder Works plant. The company was later renamed Hercules, one of the first companies to be formed by judicial decree, as part of the break-up of DuPont's monopoly in the explosives industry. By 1917, it was the most productive TNT plant in the United States. With the invention of nitroglycerine, Hercules evolved into a chemical and fertilizer company, producing formaldehyde, methanol, and anhydrous ammonia. Other plants owned by the company went on to produce rocket motors for ICBMs." The California Powder Works in Santa Cruz completely was shut down by 1916. I know the local Santa Cruz cemetery has a memorial to those killed in explosions at the Powder Works there. I don't see why it wouldn't be the same for Contra Costa County. There should be newspaper accounts of the explosions. I looked at the 1905-1929 death index for CA at www.vitalsearch-ca.com and didn't find an Alfred Hall that fit the time and place that yours died. I can't even find Nellie and Alfred Jr. in 1920 census. Did she remarry by then?