Philadelphia had the good fortune to be founded relatively late in the colonial process and it was fortunate also to enjoy early success. William Penn experienced London's bout with the bubonic plague in 1665 and the Great Fire in 1666 when he was in his 20's and he determined to build "a greene country town which will never be burnt and always be wholesome". It's unclear if people were required to build of brick or did so out of choice, but many did from the beginning - however this practice became popular, it was successful. Philadelphia was the only large colonial city never to experience a major fire.

Fired bricks - those baked in a kiln at high temperatures, were perfected by the Romans. They were the first modular, mass produced, building material and revolutionized the speed of construction. Unlike stone which requires time and a high degree of skill to dress and set each piece in its place, bricks can be handled easily by one man and laid up quickly. Since they are the same size, multiple bricklayers can work on the same wall and build the wall level and plumb. If designed properly and built correctly, a brick wall is almost permanent, requiring very little maintenance and able to withstand the harshest weather. Philadelphia was blessed with good clay deposits and sources of limestone (a local old road is called Limekiln Pike) for mortar. This is probably the place to dispel and old wives tale about brick coming over as ballast on ships. It happened, but rarely. The bricks you see are very likely to have been made locally.

Bricklayers came to Philadelphia from both England and Holland where they had developed a wide variety of brick construction styles. A brick when laid with its long side facing out of the wall is called a stretcher and when laid with the short side facing the wall's surface, it's called a header. The easiest way to lay a brick wall is to make a wall of all stretchers with each joint centered on the bricks above and below it - a so called common bond. Most walls are more than one brick deep but when multiple thicknesses are laid up and no brick is laid perpendicular to the wall's face tying the brick mass together, common bond walls are rather weak (modern common bond walls depend on metal wire ties going into the depth wall).

Brick
Brick construciton at Pompeii
The English developed a bonding pattern where every seventh or eighth course would be a course of headers which serve to tie the brick thicknesses together. The Dutch had developed another pattern where every course alternated between stretcher and header - the so called Flemish bond. While it's a very strong pattern, it is very time consuming to lay. There are many cases where the front of a building was laid in Flemish bond to "show off" and side and rear walls were laid up in cheaper English bond. In Georgian buildings, the Flemish bond may have included "glazed" headers. When they are fired, some bricks are placed in a kiln in such a way that the ends of some bricks will directly face the fire. These ends are "burned" and get a glazed coating which turns a dark blue or almost black color. These glazed headers can be used to great decorative affect. The glazed headers produce a decorative pattern that livens up the wall as the sun is reflected off of the glazed ends.