The following article is taken from The French Marans Club
ORIGIN OF COLOURING
The extra-russet-red colour comes from the impregnation of a liquid colouring on the eggshell. This liquid is produced by spongy tissues of the last 10 centimetres of the oviduct and is deposited right before laying.
Immediately after laying, a layer of mucus covering the egg dries quickly and the shell keeps its colour. It is these many mucous glands that secrete the colouring substances and tint the shell.
These substances of albuminous origin are manufactured by certain cells whose biochemical laws to date are not elucidated.
On an egg just laid and still wet, one can damage whole or part of this coloured film with one’s finger. An unhappy experiment for a hen whose egg had remained blocked in the oviduct for several days had made it possible to note that the successive layers of pigment had not ceased accumulating and formed a film in the range of 1 mm in thickness and an almost black purplish colour.
Contrary to the blue-green egg of the Araucana hen which is tinted throughout the thickness of the shell, one realizes when one breaks a Marans egg that the internal colour of the shell is perfectly white, offering an unexpected contrast, and given its colouring right before laying, is like a final signature suitable for the breed.
“An identity card” - The deposition of the pigments on eggs is not always uniform.
With the manner of a print, like a child practicing with a wet sponge soaked in colour, one observes variation in the distribution of the pigments that are unique to each layer.
The shell is uniform when the print is perfectly carried out, allowing a homogeneous diffusion of pigment over the totality of the egg. Quite as frequent as the uniformly pigmented egg is the appearance of fine networks of dots some tenth of a millimeter in size spread over a clearer background. Occasionally there can appear very brown spots, in slight relief, on a clearer or definitely more constant background.
However, these deposits of pigments are not only subjected to variations from one hen to another but also in respect to time. Observations made with trap-nests over 15 years of breeding enabled us to note in the best extra-russet-red egg layers that aspects of the shell established at the beginning of laying continue, with some variations, to some extent allotting “an identity card” for each hen.
One can thus determine 3 categories of eggshell types in the Marans:
One cannot judge the final qualities of a layer for extra-russet-red egg colour until one dozen eggs have been laid, counted from the first onset of laying. These nuances then deteriorate very gradually until the sixth month of laying. At that time the best hens lay eggs certainly less characteristically dark but still more deeply coloured than those of other breeds.
It is also noted that in a series of 3, 4, and 5 eggs laid in continuation after a one-day stop, the first egg laid is always more deeply coloured. This decline in colour intensity, and the cessation of laying during the incubation, in fact marks a natural and necessary rest of the oviduct common to all breeds As with a hen’s first egg, after a period of broodiness, of incubation or moult good layers again start to lay a superb extra-russet-red egg.
External influences on egg colour: It is significant to announce that poor sanitary conditions can considerably influence the colour of the shells. - Before any quantitative drop in production occurs, a decline in colouring, an abnormal distribution of the pigments, or the appearance of aspects which are whitish and rough are the advertisement of diseases or parasites. Excessive fattening of the hens, changes of environment, of feeding, and stress factors are also prejudicial toward loss of eggshell colour.
The thickness of the shell: -In a general way, the solidity of the shell is closely related to the factors located upstream of the production of an egg: the bird’s genetic origin, the age of the layer, feeding, breeding conditions, and the bird’s medical state.
The shell represents approximately 10% of the weight of the egg. In the Marans, when these optimum conditions are met, one notes that the solidity of the shell is greater than that of eggs of other breeds.
Even when the empirical observation is easy (when one breaks a Marans egg, it is often with a certain difficulty), no scientific proof had attested it until a group of students (promotion 1995-1997) of the Institute des Sciences et Vie de la Terre (I.S.V.T.) of Puy en Velay studied the extra-russet-red eggs of the Marans. According to measurements and calculations carried out, we could highlight which Marans hen’s eggs had a more solid shell than that of traditional eggs.
These would thus have considerable advantages for marketing, decreasing the number of potential breakages during transport, and the storage time of these eggs is quite longer than that of traditional eggs. The thickness and the solidity of the eggshell of the Marans is now scientifically shown.
It is precisely the thickness of this less porous shell with a fine and tight grain which, but decreasing the gaseous exchange, avoids the oxidation of its contents: the air penetrates with difficulty into the egg and its conservation is somewhat prolonged. The cuticle and internal and external shell membranes of the Marans egg constitute effective protective barriers against bacteria.
This lower permeability (and thicker internal membranes) often causes decreased hatchability of approximately 5 to 10% compared to majority of other breeds.
The shape of the egg - This is an inherited feature that often enters in correlation with the extra-russet-red colour of the eggs. In the best stock, one often notes a globulous form for which it its sometimes difficult to distinguish the point from the base; a distinction, however, necessary for the correct placement of the egg point down in the racks of forced-air incubators. This characteristic of an almost spherical rather than ovoid shape is to be sought because all the old writings and testimonies describe this phenomenon. Probably, the genes responsible for this characteristic are incompletely dominant.