Electrolytic capacitors use a chemical feature of some special metals, historically called valve metals, which can form an insulating oxide layer. Applying a positive voltage to the tantalum anode material in an electrolytic bath forms an oxide barrier layer with a thickness proportional to the applied voltage. This oxide layer serves as the dielectric in an electrolytic capacitor. The properties of this oxide layer compared with tantalum oxide layer are given in the following table: