It is most convenient to examine the pigment first for mechanical admixtures. If an effervescence follows the addition of hydrochloric acid, denoting the presence of a carbonate, this is generally calcium carbonate. White lead is rarely used, since it is very heavy and is more expensive. Gypsum and magnesia are occasionally added to lakes; they are recognised by examining the residue left on ignition. In testing for the metallic oxide which is united with the colouring matter a small quantity of the lake is heated in a little porcelain crucible until all organic substances are completely destroyed. The residue is then examined for aluminium and tin oxides, by means of which lakes are most frequently prepared. REACTIONS OF THE ORGANIC COLOURING MATTERS. +----------+----------+----------+--------+---------+--------+ |Colouring | Dilute | Caustic | Lime | Strong | Alum. | | Matter. | Acids. | Soda. | Water. | Nitric | | | | | | | Acid. | | +----------+----------+----------+--------+---------+--------+ | YELLOW.| | | | | | |Buckthorn |Slight |Yellowish |Slight |Reddish |Pale | | berry |turbidity.| red. |greenish|brown. |yellow. | | extract | | |precipi-| | | | | | | tate. | | | | | | | | | | |Dutch pink|Yellow |Brown. |Brownish|Reddish | —— | | |solution. | | |brown. | | | | | | | | | |Weld |Slight |Golden |Greenish|Golden |Slight | | extract |turbidity.| yellow. |yellow. |yellow. |yellow | | | | | | |precipi-| | | | | | | tate.| | | | | | | | |Weld lake |Yellow |Unaltered.|Greenish|Golden | —— | | |solution. | |yellow. |yellow | | | | | | |solution.| | | | | | | | | |Gamboge |Unaltered.|Deep |Deeper |Deep | —— | | | |reddish |yellow. |yellow. | | | | |yellow. | | | | | | | | | | | |Gamboge |Yellow |Deep |Deeper |Deep |Yellow | | lake |solution. |reddish |yellow. |yellow. |precipi-| | | |yellow. | | | tate.| | | | | | | | |Fustic |Becomes |Dirty |Deeper. |Slight |Pale | | |paler. |brownish | |precipi- |yellow |