Why did bare wire electrodes produce brittle welds in early arc welding?

In early arc welding, the use of bare wire electrodes resulted in brittle welds primarily due to the lack of protection from the atmosphere during the welding process.12

When metals are heated to a molten state, they react rapidly with oxygen in the air, causing the metal to oxidize. In the case of bare wire electrodes, molten-metal droplets became coated with these oxides as they transferred across the arc. When these oxide-coated droplets were entrapped within the weld pool, they compromised the integrity of the joint, leading to brittleness. Furthermore, this exposure to air caused valuable alloying elements to react so quickly that the final composition of the deposited metal differed from the original electrode material.1

To solve these metallurgical issues, engineers developed fluxes and inert atmospheres to displace air and prevent oxidation, eventually leading to the complex mineral and metal coatings found on modern electrodes.12