The arc stabilizer for AC is potassium, while sodium is used as the arc stabilizer for DC. Not all electrodes are created equal, not all electrodes can be used with both AC and DC. Most E7018 electrodes are intended to be used with DC, however, some E7018 electrodes are formulated to run with either AC or DC. The container often indicates whether the electrode can be used with DC only or AC and DC.
Inexpensive "buzz boxes" are typically fine for welding with AC and low open circuit voltage. DC electrodes usually require higher open circuit voltage to provide easy arc initiation because of the higher ionization voltage required by sodium.
The voltage under load, i.e., while welding, is a function of the rated amperage of the welding machine. The formula is V = 20 + 0.04X Amperage. This is the case if they meet the requirements of the NEMA. If they don't comply with the NEMA, all bets are off.
A 200 amp machine will only provide 28 volts while welding. A 300 amp machine will provide 32 volts. One must consider both the amperage required to weld with a rod of a certain diameter and the arc voltage while welding. In addition, the length of the welding leads, oxidized connections, etc. add to the voltage needed to provide the proper conditions to weld successfully.
Interestingly, I find the E6010 usually runs with a higher arc voltage than an E7018 of the same diameter because most E7018 is intended to run with a short arc length.
Good luck - Al
Does the machine have overlapping amperage ranges? In other words, a finite/limited wattage machine. Volts high amps low or versa vice Volts low amps high. If so, the highest OCV will be found by setting the machine at the lowest overlapping amperage range with the required welding amperage adjusted towards the higher end of the fine current adjustment. This can be checked with a handheld voltmeter and ammeter.