De Genettes wrote as follows:
"Most of the organs represented by colored waxes are cast in plaster moulds formed directly from the natural organs. The finishing touches are then added by a skilled sculptor who works side-by-side with the actual cadaver. The sculptor always works under the supervision of an anatomist, because even the most excellent sculptors would be unable to reproduce nature accurately without such guidance. Any organs that cannot be cast directly are modelled in clay or wax by artists who are extremely skilled in this type of work and copy from the cadaver itself. A plaster impression is then cast on these models. This technique is used particularly for statues of whole figures.
When a plaster cast is to be made for an anatomical statue, a sculptor is firstly commissioned to produce a full size wax model. This is copied from life, nude and in the position the anatomist finds that the organs or parts to be represented are shown to the greatest effect. This first stage takes about six months. When complete, separate models of individual organs must be made once each has been dissected; and the entire process must be carried out under constant supervision and guidance."

Before proceeding to the second stage of the work, the wax used to model the anatomical parts had to be prepared. In the Florentine laboratory, from the end of the Eighteenth century to the second half of the Nineteenth century it was common practice to use a mixture obtained by combining top quality beeswax with wax from other insects. This Chinese wax has a high boiling point and is produced by certain types of plant parasite. The resulting mixture was poured into a big cauldron made out of tin-plated copper and then left to melt over a very low heat. Lard (animal fat) and sperm-oil (plant oil) were added to the mixture from time to time as required.

The second stage involved the production of a plaster cast which, when set, formed a negative impression or mould, which was kept so that the same piece could be reproduced over and over again if required.
The mould was generally cast directly from the anatomical part, which was spread with a light layer of fat to protect it from damage during the operation.

Now came the third and most delicate stage: the production of the wax part. The success of this stage depended upon the skill of the artist - and the artists employed at the Specola Museum were unsurpassed in their proficiency. Once the wax mixture had melted, a dye was added which had already been dissolved in turpentine with the addition of small quantities of powdered white lake. Natural dyes such as madder lake, cinnabar and granulated white wax were used to obtain the required muscle colors. Before the melted wax was poured, the plaster cast was soaked in warm water and its inner surface was spread with soft-soap. This made the plaster less porous so that the model came away more easily. The hot wax mixture was now poured into the mould. The operation was carried out by pouring the wax in successive layers, each at progressively lower temperatures.
Once the wax was cool, the mould was opened and the next stage could begin.

The fourth and final stage involved cleaning the resulting wax cast. This was carried out by scraping any imperfections off the mould and polishing the model surface with a soft brush soaked in oil of turpentine. The muscle striations were then produced using heated iron implements of various shapes and sizes. Blood vessels, nerves and lymphatic vessels were then added: arteries, veins and nerves were produced using wax-coated wire or thread. The finer vessels were painted in using a very fine brush and colors were touched in as required. The surface of the model was then covered in a coat of clear varnish. This glaze added gloss to the model, protected it from dust and muted the colors.

* taken from:
Various Authors,
Le Cere Anatomiche della Specola [Wax Anatomical models of the Museo della Specola or Observatory Museum], Florence 1979