Final answer:
Wax is used to extend the borders of an alginate impression tray due to its malleability. Other materials like plaster, silicone, and alginate are not commonly used for this specific purpose.
Step-by-step explanation:
The material often used to extend the length or depth of the borders of an alginate impression tray is wax. Wax is pliable and can be easily shaped or molded to the contours of the tray, making it ideal for customizing the tray's borders to ensure a complete and accurate dental impression.
Other materials such as plaster, silicone, and more alginate are not typically used for this purpose because they either lack the same malleability or they are the impression materials themselves.