The bell was ordered in 1751 by the Assembly of the Province of Pennsylvania. It was cast in England by Thomas Lester and delivered in 1852. It cracked upon the first ringing in Philadelphia, and two local workmen, Pass and Stow, recast it. They increased the copper content by 10%, but the resulting bell had poor tone. The recast it again, adding 1/4% silver. This is the final casting that still exists today.

The bell was regularly used for over 80 years, tolling for the opening of the Assembly and other functions. The bell cracked in 1835, reportedly while tolling for the funeral of Chief Justice John Marshall.

The reasons for the crack, as given by A. R. Rosenfeld [1] were:

- Too high percentage of tin, known to cause brittleness. The original bell (before recasting) is believed to have been 26% tin, a percentage known to be brittle and prone to cracking. This may have contributed to the 1835 crack as well, particularly if different portions of the bell have different proportions of metal.

- Tuning the bell by removing metal at the rim end, which left a "ragged rim" that concentrates stress

- The bell was constantly overloaded during its lifetime. It is known to have experienced one drop/fall, but more importantly, it was constantly overloaded when rung.

The force striking large bells such as this one must be carefully modulated. The Liberty Bell was constructed so as to be swung--a method of ringing, which (if done correctly) reduces the force of impact. But the bell was not rung by swinging, but by clappering.

In clappering, a rope is directly attached to the clapper. Pulling the rope causes the clapper to strike the bell, which is fixed. This "direct strike" method of ringing the bell means there is essentially no limit to the force of the strike. If the ringer pulls the rope gently, a soft strike will result. If the rope is pulled with great force, a very forceful strike will result.

Rosenfeld believes that a lifetime of too-forceful strikes and other overloads, such as the fall, perhaps combined with the other two factors (ragged rim and too-high tin content) led to the crack in 1835.

For the birthday of George Washington in 1846, an attempt was made to make the bell ringable again. Workmen drilled out the then-existing crack--a technique called "stop drilling". I believe that the two bolts or rivets were placed at that time--one near the bottom and one near the top of the stop-drill area.

With such a long stop-drill, allowing the two sides of the stop-drill to move independently would have inevitably led to stress at the upper end of the stop-drill and further cracking from that point. Anchoring the two sides of the stop-drill together near the top and the bottom was an attempt to stabilize the lengthy stop-drill to allow the bell to be rung again while minimizing the chance of further cracking. (Note that I cannot find specific documentation indicating when the two bolts where placed, or that they were placed at the time of the original stop-drill. It is possible they were put in place as part of later attempts to stabilize the bell and prevent further cracking. However, given the extreme length of the stop-drill repair it seems very unlikely that any attempt to ring the bell would have been made without some method of stabilizing the two sides of the stop-drill. This makes it likely that bolts similar to those we see today were placed at that time. The bolts you see today may be those original bolts or perhaps later replacements.)

In this photo (National Science Foundation, Office of Legislative and Public Affairs), which shows a view of the Liberty Bell from the inside, you can see the drill marks from the 1846 stop-drill as well as the two bolts, one placed near the bottom and one near the top of the stop-drill along the original 1835 crack.

(You can also see the inside framework, designed to support the bell and stop further cracking. Various frameworks have been in place, in an attempt to stop further cracking as the bell traveled the country several times in the late 1800s and 1900s.)

The stop-drill repair allowed the bell to be rung a few times during the 1846 birthday commemoration, but by the end of the day the crack had extended from the stop of the stop-drill section. Stop-drilling is a legitimate crack repair technique, but with such an extensive crack, it was inevitable that further cracking would occur under the stress of ringing.

Since 1846, the bell has not been rung again with the clapper. On a few occasions it has been rung by tapping (lightly) with a mallet. However, considerable further cracking has occurred during travel as the bell went to various destinations via railway. The inner framework was designed to support the bell and limit further cracking, and in recent decades extensive travel has been stopped.

If you examine photos of the bell carefully--such as this one from Wikimedia Commons--you will see what is commonly considered the "crack" in the Liberty Bell is actually the result of stop drilling along the 1835 crack. The result of the stop-drill operation is considerably wider and more visible than a mere crack would be. The extensive cracks emanating from the top of the stop-drill are visible as well, if you look closely.

Sources:

[1] Rosenfield, A.R. The crack in the liberty bell. International Journal of Fracture 12, 791–797 (1976). https://link.springer.com/article/10.1007%2FBF00034619

This is the primary source for the narrative and conclusions above.

[2] Liberty Bell Passes Stress Test, National Science Foundation, Office of Legislative and Public Affairs, https://www.nsf.gov/od/lpa/news/03/pr0337.htm

[3] The Liberty Bell, National Park Service, https://www.nps.gov/inde/learn/historyculture/stories-libertybell.htm