Here I think this addresses it:
If We Wish to Think it is a Fake Picture of Jesus?If we want to believe that the Shroud is not genuine then we have to consider some basic questions. How did the faker of relics accomplish this.
How did a faker of relics alter the chemical properties of the carbohydrate coating to create the color and how did he do so with such artistic precision -- on both sides of the cloth?
The history of art is the story of the evolution of styles, techniques, methods and technology. Every work of art and fakery is no exception. Every form of art and craft has precedents. When a new technique is discovered it is exploited. Over time the technique is refined and improved. Where are the precedents for pictures such as those that we find on the Shroud? Where are the other works in this new-found technology? Are we to imagine that some genius invented a new way to create pictures, that a single picture was made and the technology was lost to history?
How did he create a suitable negative picture hundreds of years before the discovery of photographic negativity? How did he know that he had it right? How, without a camera and film, could he test his work? The negativity is extraordinarily precise and correct. Was he simply lucky?
The bigger question is why? What was his purpose? What was his motive? If we are to ask why he created an extraordinarily complex chemical picture, in negative, we must ask some other questions.
Why a negative image when a positive image would be more convincing -- keep in mind that gradual tone negative images were unknown?
Why did he go against conventional expectations of his era? Why did he create a picture with wounds from nails that went through Jesus' wrists? All art and all expectation throughout medieval Europe showed Jesus nailed to his cross through the palms of his hands.
Why is Jesus shown completely naked, unlike in all artistic depictions everywhere throughout the history of Christianity?
Despite many attempts to do so, no one has found or invented an artistic or crafty technique that can reproduce even a few of the characteristics of the images. But that does not mean, that in the future, someone will not find a method to create such images. But if someone does so, the tenacious question will remain: How likely is it that there would be such a one-of-a-kind work of art for which there are no known precedents; created by methods that were never again exploited?
Any method that might be devised must be scientifically credulous, fit into the history of art and conform to the cultural expectations in which the technology was supposedly employed. If not, it will be seen as newly invented art designed to mimic an otherwise unexplained natural process or a supernatural event. The skeptic has a dilemma. To believe that the Shroud is fakery he or she must rely on an underlying belief that transcends scientific fact.