For those who have looked into professional CD manufacturing, you could not have failed to come across the term “Glass Mastering.” We have been pressing CDs for decades now, but glass mastering still seems to be the most misunderstood phrase in the industry. To be honest it is not only the novice who is confused, many music industry long timers still only have a vague idea of what the process involves.
In this blog I hope to dispel the mystery that surrounds the glass mastering process. If you are looking for detailed technical specifications on the process you may have to look elsewhere, this post is only going to cover the basic process in order to keep it short.
“What is glass mastering?”
Glass mastering is used to describe the process of transferring data from a CD/DVD Master to a metal stamper that is used to”press” the discs.
“So why is it called glass mastering?”
The first stage of Glass Mastering is to etch the 0’s and 1’s that make up the data on the master on to a piece of photosensitive film that has a glass disc as the substrate (base). This is done by a machine called a Laser Beam Recorder, this is a huge and highly accurate version of the CD recorder in your computer. This glass disc with the information etched onto the film is then sent to galvanic processing to create a metal version, a stamper, of the information recorded on the photosensitive film.
“Why is it made of glass?”
The reason for using glass is that it is possible to get glass polished to a very smooth surface. This is vitallas any imperfections in the base will cause errors in the CD manufacturing process. It is important to note that at every stage of making the stamper, each part is checked against the original CD master to ensure that it is an exact clone.
“What happens to the glass master after you make the stamper?”
After galvanic processing when we have a stamper, the glass substrate is washed down and re-used to make other stampers.
“As I have paid for glass mastering, can I keep the glass master?”
Afraid not, as we re-use the glass substrate to make other stampers. It would be a bit like ordering a cup of coffee from a cafe and then asking to keep the kettle that boiled the water.
“Can I keep the stamper?”
Generally speaking, no. The stamper contains nickel, this is a valuable material, and once we are finished with the stamper it is melted down and re-used. Beside this it would not be of much use to you, all the factory’s I know of would not be happy using a stamper that has been glass mastered by another CD pressing plant. In most cases the stampers are incompatible with the machines used in other plants. And due to the large amount of quality checks made throughout the manufacturing process the pressing plants prefer to work from their own stampers.
“How long do you keep the stamper, what if I want to re-press my CD?”
All factory’s retain the stamper for different periods of time for future re-presses. Most will keep the stamper for at least 3 years from the date of the last pressing and some will hold it up to 7 years. However these days pressing plants will keep a digital copy of the master stored on hard disc, so if after this time you want a re-press and they no longer have a stamper they can re-glass master the project.
“But my mastering studio has given me a glass master, why do I have to pay for glass mastering”
This is a common misconception. Mastering studios create “CD masters” and not “Glass Masters”, it is the CD master given to you by your mastering studio that is used to create the glass master.
“Can you send my CD master back to me after you have made the stamper?”
Of course, this is your master, and once we have made the stamper and kept a copy of your CD master on hard disc, we no longer require the original master. One thing I will say that if you are looking to keep a safety copy of your disc, I would recommend keeping a factory pressed copy of the CD in a safe place. These are less likely to degrade over time, and will generally have lower errors compared to a CD-R master.
“Is it the glass mastering process that makes pressed CD’s better than CD-R duplication?”
Not quite, however it is the meticulous attention to detail in the glass mastering process that sets the stage for high quality CD pressing. There is much more to it than creating a high quality stamper, the two types of discs differ in many physical ways as well. To list all the differences is beyond the scope of this post, and it is an issue I would like to devote a an entire future post to. At this point I will just say the two formats are quite different in many ways, and CD duplication is not to be confused with high quality CD manufacturing.
So essentially that is a basic overview of glass mastering for CD manufacturing. Some of the points I have made may not apply to all CD pressing plants, so if you are not using Pure Music to manufacture your CD, I would check with your supplier directly with regard to how long they will keep the stamper and if they hold a version on hard disc for the future.
I hope you have enjoyed this post, and as always, if you have any questions regarding this article, feel free to drop me a line in the comments section, and I will do my best to give you an answer.