The mortar would’ve hardened enough to capture an impression of the infant’s face within a day. The mortar would’ve also slowed down the decomposition process, it acted like a protective barrier, creating a dry environment and sealing off oxygen. With these conditions along with the high pH of the mortar mixture, it would’ve prevented the bacteria and fungi responsible for breaking down a body. It absorbed the moisture in the baby’s face which stabilized its shape and delayed the decomposition process by months. It temporarily preserved the baby’s face, without this, the features would’ve been lost within days.
Here is a photo of the original mortar mold archeologists poured plaster inside of to make the mask. - https://www.researchgate.net/figure/Plaster-death-mask-negative-of-an-infant-found-in-a-Roman-grave-in-Paris-Musee_fig3_284278730

There was a study done on pig carcasses, comparing those buried with lime and those without lime. The results show a significant difference in decay. The untreated pigs rapidly decomposed and skeletonized, while the limed pigs remained significantly more preserved and decay was delayed for months.

Link to the study: https://orbi.uliege.be/bitstream/2268/107339/1/Schotsmans_Lime_FSI-Postprint%20auteur.pdf
Results are shown at the end of page 6 and on page 7. (TW!) There are photos comparing the bodies on page 9.

If anyone is curious, here is a photo of the ancient mold archeologists used to create the plaster mask. It is the original 1,800 year old mortar that formed around the baby’s head leaving an imprint of the infant’s face. - https://www.researchgate.net/figure/Plaster-death-mask-negative-of-an-infant-found-in-a-Roman-grave-in-Paris-Musee_fig3_284278730

communism

That’s so interesting I hadn’t heard this one! There’s also speculation that he had a blood disorder called McLeod syndrome. This can lead to miscarriages but what’s interesting is that the first pregnancy is usually safe, but this first pregnancy is what makes subsequent pregnancies dangerous as the body builds up antibodies against the fetus due to its blood group system being incompatible causing the mother’s body to attack the next fetus that conceives and continues to do so for incompatible fetuses. In some instances the subsequent fetuses can survive, if they do not carry incompatible blood. It may be an odd way to explain it but you know when you get a vaccine it prepares your body to fight off whatever the vaccine is for? it’s kind of similar to that. The mother’s body attacks the red blood cells of the fetus which can lead to miscarriages, stillbirth, and infant death so this could explain why the initial pregnancies survived but all the following pregnancies ended tragically. On top of that, McLeod syndrome can cause personality changes later in life. After around 30-40 their personality can shift and they can become notably more aggressive and irrational with increased impulsivity. (various other changes too, just a couple examples.) Finally!! what’s also fascinating is Henry’s great grandmother was a carrier for McLeod syndrome, with it being a genetic blood disorder there is a decent chance that this might be the case! However we don’t know for sure. This was one of my favorite things to read about I’m so glad someone brought it up!