Thickness controls the width of the arc line in pixels. This value can be increased to make the line arc thicker.
Thickness can be increased to create a wedge.
Note that Thickness can be increased to any value including values larger than the radius of the arc. Large values can result in the arc rendering past its center point creating a bowtie shape.
Other undesirable rendering effects can happen with large thicknesses when working with arcs which have one size (width or height) larger than the other, as shown in the following image:
