Burtonian line around the gingiva is caused by the absorption of:

Oral Manifestations: The formation of a ‘lead line’ similar to the 'bismuth line’ occurs in lead poisoning. This gray or bluish black line of sulfide pigmentation occurs in the gingiva, but is somewhat more diffuse than that of bismuth. It is also found occasionally in other areas of the oral cavity. Ulcerative stomatitis is an additional reported finding. Excessive salivation and a metallic taste are also common complaints in this condition, as is swelling of the salivary glands. It is reported by Altshuller and his associates that lead is deposited in the deciduous teeth of children suffering from lead poisoning, and that those teeth may serve as an index of the body burden of lead. Key concept: Burton's line, also known as the Burton line or Burtonian line, is a clinical sign found in patients with chronic lead poisoning. It is a very thin, black-blue line visible along the margin of the gums. Ref - Shafers 7th Ed P:558