What we have is the circular font and a rectangular secondary basin. They are both in very bad conditions.  

  
Reconstruction of the two basins 

The baptismal font itself consisted of a circular basin with an internal diameter of about 3 and a half meter. A circular brick wall, 65 cms thick and clad with marble, was the rim of the font. But this wall was also damaged in the fifteenth century. How high was it? Perhaps about one meter? Its inside was clad with white marble and bluish cipollino marble. One of these cipollino marbles is a reused funerary inscription of a woman called Hilarina, who died in the fourth century. This circular font must be were solemn baptism was celebrated on Easter and Pentecost.   

  
To the east of the circular font, there is a rectangular basin, which measures about 2,2 x 1,35 m. On the south short side, there is a kind of nich with the bottom 15 cms above the bottom of the rectangular basin. Also this basin is damaged. What was its original shape? Were the walls high all around? This would make no sense considering the nich, which should have ended as a kind of small apse. Perhaps one can compare with some earlier Roman nymphaea from Pompeji with a kind of aedicula with a nich in front of a low basin, and reconstruct the two basins as in this drawing.  
  
The function of the secondary basin 

We have no clues as to the function of the secondary basin, except that it clearly contained water.  

Was it used for the washing of feet? Some sources mention this rite after immersion and anointing. St. Ambrose in Milan wrote in 380-390 that this part of the rite had been left out in Rome . For this reason, the secondary basin in San Lorenzo in Lucina could be used for the washing of feet only if the baptistery was built some time before Ambrose wrote, in the mid fourth century.  
 
 

Ambrose writes:  "The Church of Rome does not have this custom, whose character and form we follow in all things. Yet it does not have the custom of washing the feet. perhaps on account of the multitude this practice declined."  (De sacramentis 3,5: Sources chrétiennes 25)

 

Another possibility is that the secondary basin was used for the blessing of the water before the baptism. Some martyr legends, the so called "Passiones", written i Rome in the fifth century, describe how baptism was celebrated in Rome in that period. And they often mention that as there seldom was any running water, the water was first brought - allata est aqua - and then blessed. The water would then have been taken from this basin after the blessing and used for the baptism in the bigger font.  

Later history of the baptistery 

This fifth century baptistery was used in a thousand years. It was rebuilt several times.  

The circular font had one or two steps added on its west side, perhaps in the general reconstruction of the chucrh in the eight century, or already earlier. Probably this step was meant for the priest who baptized and who didn't want to get his feet wet. Such steps can be found also in the churches San Crisogono and Santo Stefano in Via Latina in Rome.  

The big reconstruction of the entire church took place after its destruction by the Normands in 1084. Probably this is when the bottom of the circular font was covered with a kind of plaster called cocciopesto, and the secondary basin was completely covered. The hole in the middle probably shows that from now on, baptism was celebrated in a smaller font of the modern kind as we are used to today.  

All the remains of the old baptistery were covered up by the floor of a new chapel built in the fifteenth century. An important tomb destroyed part of the circular font which no longer was visible. M E Bertoldi understood that this was the chapel of St. John the Baptist, mentioned in the old documents, which means that the chapel still was used for baptism for some centuries.   

This is what archeaeology tells us about this baptistery. This was one of the many, perhaps 25 smaller baptisteries in the tituli, created between the fourth and  sixth centuries for the baptismal celebrations of Easter and Pentecost. In the beginning of the fourth century Rome had only one baptistery at the Lateran, but the religious transformations in the fourth century made that baptistery insufficient for the great masses of converted adults and their children after them. Together with other baptisteries in titulus churches, like those recently found in San Marcello al Corso, Santa Cecilia in Trastevere, San Marco and San Clemente, this baptistery reflects the Christianization of the city of Rome: one of the most interesting transformations in the past, and one which helped creating the European culture, that culture which unites Sweden, Italy and great part of the modern world.   
 
 

Would you like to know the exact date of Easter baptism in the any year after 325? Check the site "Calculation of the Ecclesiastical Calendar" by Marcos J. Montes. Enter a year after 325 CE and a script will calculate the day of the Easter in the Western and Eastern Traditions, the movable feasts, and other significant liturgical celebrations.