Le immagini e il testo inglese sono attinte dal sito NGK Spark Plugs

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	Normal Condition An engine's condition can be judged by the appearance of the spark plug's firing end. If the firing end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning optimally. Condizione Normale Gli elettrodi sono di colore bruno e grigio chiaro
	Dry and Wet Fouling Although there are many different cases, if the insulation resistance between the center electrode and the shell is over 10 ohms, the engine can be started normally. If the insulation resistance drops to 0 ohms, the firing end is fouled by either wet or dry carbon. Incrostazioni / Sporco / Residui - asciutti o bagnati Possono verificarsi vari casi: - se la resistenza dell’isolamento tra l’elettrodo centrale e l’involucro, è superiore a 10 ohms, il motore si avvia normalmente; - diminuendo la resistenza, gli elettrodi si incrostano di residui asciutti o bagnati.
	Overheating When a spark plug overheats, deposits that have accumulated on the insulator tip melt and give the insulator tip a glazed or glossy appearance. Surriscaldamento Quando una candela si surriscalda, i depositi accumulatisi all’apice dell’isolamento, fondono e l’apice dell’isolamento appare smaltato
	Deposits The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and the engine's operating duration. Depositi L’accumulo di depositi può dipendere da dispersione d’olio, qualità della benzina e dalla durata della fase
	Lead Fouling Lead fouling usually appears as yellowish brown deposits on the insulator nose. This can not be detected by a resitstance tester at room temperature. Lead compounds combine at different temperatures. Those formed at 370-470°C (700-790°F) having the greatest influence on lead resistance. Residui di Piombo Appaiono sotto forma di depositi giallo/scuro sull’isolamento
	Breakage Breakage is usually caused by thermal expansion and thermal shock due to sudden heating or cooling. Rotture Sono solitamente causate da “shock termico” conseguente ad improvviso Innalzamento della temperatura
	Normal Life A worn spark plug not only wastes fuel but also strains the whole ignition system because the expanded gap (due to erosion) requires higher voltages. Normal rates of gap growth are as follows: Four Stroke Engines: 0.01~0.02 mm/1,000 km (0.00063~0.000126 inches/1,000 mi.) Two Stroke Engines: 0.02~0.04 mm/1,000 km (0.000126~0.00252 inches/1,000 mi.) Durata media / Usura degli elettrodi à Aumento distanza (motore a quattro tempi) 0.01~0.02 mm per 1.000 Km (motore a due tempi) 0.02~0.04 mm per 1.000 Km
	Abnormal Erosion Abnormal electrode erosion is caused by the effects of corrosion, oxidation and reaction with lead - all resulting in abnormal gap growth. Erosione anomala L’erosione anomala dell’elettrodo è dovuta solitamente a effetti di ossidazione e reazione al piombo
	Melting Melting is caused by overheating. Mostly, the electrode surface is rather lustrous and uneven. The melting point of nickel alloy is 1,200~1,300°C (2,200~2,400°F). Fusione Causata da surriscaldamento. La temperatura di fusione della lega di alluminio è 1,200~1,300°C (2,200~2,400°F).
	Erosion, Corrosion and Oxidation The material of the electrodes has oxidized, and when the oxidation is heavy it will be green on the surface. The surface of the electrodes are also fretted and rough. Erosione, Ossidazione, Ruggine
	Lead Erosion Lead erosion is caused by lead compounds in the gasoline which react chemically with the material of the electrodes (nickel alloy) as high temperatures; crystal of nickel alloy fall off because of the lead compounds permeating and seperating the grain boundary of the nickel alloy. Typical lead erosion causes the surface of the ground electrode to become thinner, and the tip of the electrode looks as if it has been chipped. Erosione da Piombo Causata dal reazione del piombo con la lega di nickel degli elettrodi.