Using a deep saucepan, fry salt pork slowly, remove and add diced onion. Cook on low heat until onion is soft. Add clam juice, diced potatoes. Cover pan, bring to steaming point. Lower heat and cook at least 15 minutes. Remove cover add clams including juice. Cook about 10 minutes with cover off, allowing clams to simmer along in cooked potato mixture. Add milk and butter heat to serving point. Taste add salt and pepper as needed.