A mark enhances a girl's beauty, it marks her emotionally, and instills into her psyche the inescapable idea that she is truly a slave.
On Gor, a girl who is slave will be branded, but true brands on Earth are actually rare. They can be larger and clumsier than what is described and implied in the books. Earth slaves should certainly be marked, this is not a temporary relationship that can be erased. The mark will eternally show that she is a slave. How a slave is marked though may vary, both in technique and in symbol.
There are of course other ways to mark a girl. A collar is an obvious one, addressed elsewhere. Piercing is one that has become common in the mundane world to some degree, but does not mark the girl and her psyche to the depths that are possible with other methods.
On Gor, the vast majority of slaves are branded, marking them nearly permanently as slaves. Slavers will only rarely sell a girl who has not yet been branded. In some cities, it is even illegal to offer an unbranded girl for public slave. The actual act of branding is commonly done by a trained Iron Master. Many Goreans prefer a girl to be hand branded, but some slavers use mechanical branding devices that result in uniform marks.
On Earth, the vast majority of slaves are marked in some fashion, but there is no uniform approach. Numbers from The Slave Register, the word slave in kanji or in english script, and a few with actual Gorean marks. Marking usually takes place as a tattoo rather than a true brand, and each occurrence is truly unique.
There are three standard marking places: left thigh, right thigh, and lower left abdomen, though upon the left breast or behind the left ear is also possible. The most common site is high on the left thigh, high enough to be covered by even a brief tunic, however a slave may be branded anywhere upon her body.
Though there have been examples of fine, well done brands, in general what is described in the books is not possible on Earth. When you take a hot brand, a piece of metal heated in a normal fire, and apply it to a human's flesh, you do not get the pretty brand implied in the books nor the brand people are used to seeing upon a horse or cattle. The flesh of the slave will heat beyond just where the brand touches, the scar will be larger than the metal brand, forming more of a blob then any kind of detailed mark. Most brands currently done, for sororities and fraternities mostly, are large simple shapes.
About the only fully successful branding I have seen was a very complicated affair to set-up. The brand was made in the desired shape, and then an anti-brand was created. A large block of steel was taken and a negative of the brand was cut out of it, allowing a hollow space through which the brand could be pushed. The anti-brand was placed in the freezer, and the brand in a gas fire. When the two were at their absolute temperatures, they were applied. The anti-brand first, against the skin of the thigh. The hot brand was then pushed very quickly through the hole in the block of steel until it pressed hard against her flesh. Both were then quickly removed. This branding was very successful, and the reason why is that the brand was not able to cook the flesh beyond where it was actually in contact with it. The brand thus was finer, and did not become a large mass.
Based upon the successful anti-brand application, i wonder if a simpler approach would work almost as well. Instead of cutting the negative from the block, simply apply the entire block on the branding site, leaving it there long enough to cool all of the flesh. Then when the hot brand is applied, it will certainly heat the skin it comes in contact with, but likely not much beyond that area, or at least only slightly so.
These I have yet to try, but likely will sometime.
Tattoos certainly allow for the greatest level of detail, the finest designs, and also are much easier to have done as there are practitioners everywhere. But what they gain in ease and appearance, they lose for having a non-tactile nature. Running your hand over that area, you cannot feel the mark like you do with other techniques. Further, their application is relatively painless and doesn't give the girl the impact of that moment to quite as great a degree.
Tattoos do not need much description because they are so common but they remain a valid option.
This is the method I will do with my next girl, which I think comes closest to what the books describe in effect, though not in application. They do retain the value of the pain of application, and later after healing will also have that tactile feeling. Depending on the skin tone of the girl, they can be naturally very subtle or they can stand out a great deal from the rest of her flesh.
The pattern is drawn upon the girl, and then with a scalpel, the pattern is incised through all of the levels of flesh. Next, the skin within the incisions is peeled and removed, leaving the pattern open and exposed. If the girl is very light, or a dark mark is desired, ink or ashes can be applied to this opening. As time goes on, this wound will heal. The longer it takes, the more clear the scar will be.
It is said that piercing the ear is an act of claiming, similar to that of collaring a girl. I suggest that this once might have been true, but is no longer. In John Norman's time and before, a pierced ear was tantalizing, as an ankle revealed might have been in earlier times. Now, it is common even for children to have their ears pierced as young as one. For a similar psycological effect as a pierced ear might be to a Gorean, I would suggest the piercing of a nipple, clit hood, or labia.
This is not to say that a beast should not have pierced ears, or even a pierced nose or belly. Merely because it has become common is not necessarily a reason not to do a thing. A piercing is not solely done as an act of claiming, it is also about attraction. A pierced girl can be attractave and provocative.
What to mark the girl with, something traditional or custom, something large or small? There is value in each of these I think, but I will relay what I believe.
A kef is essentially a fluid lowercase k that represents the k in the word kajira. When used in a scarification, it must be relatively simple, though more detailed than a brand. When used in a tattoo, it can be extremely detailed.
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I have a kef that I like, that is custom, designed by mean with meaning behind its variation from the standard, and so it is special, but it remains a kef, and so has the traditional and obvious meaning.