"Unlined" means that their interior is made of copper (no nickel, tin, stainless-steel nor chemical lacquers). Pure copper tarnishes with prolonged exposure to liquids and with moisture in the air. It is normal.
CLEANING: As soon as you have enjoyed your drink, hand wash the mug with warm water, mild soap and a soft sponge. Dry with a soft towel, then dry again with a second dry towel.
KEEPING TARNISH TO A MINIMUM: Your mugs will develop some tarnish naturally. The best way to reduce the occurrence of tarnish is by handwashing the mug soon after use, and by not allowing drinks to sit in the mug for long periods.
REMOVING TARNISH: If tarnish spots do develop, apply a home-made acid such as lime juice and table salt for 2-3 minutes (avoid using chemicals to remove tarnish), then wipe thoroughly with a soft towel. It is important, that once more, the mug is hand dried twice so no moisture stays behind.