The debate of whether scratchboard work needs to be protected with glass is not one that is easy to answer and not one that everyone will agree on. Without a doubt our artwork looks its all time best in the natural state. You can take in all the fine details and every little tiny line best without anything between you and the board.
However, is this best for the art? A few important aspects need to be considered.
1) the Lightfastness of the material- does it need UV protection from UV protecting glass? - The answer to this one is that colored inks are more prone to fading than black. Black ink is typically made primarily with carbon as the pigment and carbon is very resistant to fading even in direct sunlight. The black ink has its own natural UV protection so for black and white scratchboard UV protection via glass is not needed. Fine art should never be hung in direct sunlight to maximize longevity no matter what medium, but black and white boards should not be as affected as most other mediums by sunlight. Color inks are not as light fast and would benefit from additional UV protection.
2) Resistance to incidental scratches - Here things get a bit more complex. Most artists spray scratchboard with multiple coats of a fixative varnish to protect the work once they are done. Once it is sprayed it will not scratch as easily or accept additional inks. The sprays also break down oil from the artists hand to give a nice clean look to the board. How many layers of fixative a piece needs to protect it is debatable- but in most cases a minimum of 4-6 layers should be used. This makes it much harder to scratch through the spray and down into the clay through incidental scratching.... however it is still possible! Something very abrasive rubbed against the board could still damage it. And the spray layers themselves are susceptible to being scratched and will show scratches if this occurs. While there is no real damage to the board and it is fairly easily repaired by additional spray (sometimes a bit of sanding of the top layers of spray can be required if the scratch is deep) most collectors and owners of original artwork will have no clue how to do this or what type of spray and it is simply an inconvenience. Areas of solid black will show scratches into the fixative more visibly than scratched areas.
3) Resistance to damage from oils and acids- I do not have all the answers on this one either. For example if spaghetti sauce accidentally splashed up onto artwork without glass and was not noticed for a month, could it be removed without damage when it was noticed? Would the acidity from the tomato sauce eat through the fixative? Even after it was cleaned would the oils leave a noticeable ring? I have used glass cleaner to clean a sprayed board and it worked well, but I was only cleaning a water based material from the board. Glossy finishes seem to clean more easily than matte finishes in my experience.
4) Where will the art be hanging? - This can be a key deciding point in my opinion. If the art is to hang in a high traffic area or an area where there is a reasonable chance it will be brushed against or have something sprayed or splattered on it I would always recommend it be protected behind glass. If it will hang in a highly protected area or one where there is very little change of damage then it is probably ok without being behind glass.
I know some scratchboard artist that have for years hung their original work without glass. They have had no problems and feel confident that their work is safe without glass. But for me, I suggest museum glass to all of my collectors. Yes it is quite a bit more expensive but presents the artwork is such a fashion that the glass is not a distraction (very minimal glare) as well as providing additional UV protection. When you look at work behind museum glass it is almost like there is no glass at all. standard glass, while the least expensive, provides no UV protection and can catch a lot of glare which is very distracting. Non-glare glass has a frosted textured look which can be almost opaque at certain angles. It does provide some UV protection, but the frosted look can be very distracting and totally obscure the details of the artwork. So in my opinion museum glass is the only way to go for long term presentation.
Shipping scratchboard behind glass can be very scary and if the glass breaks it can certainly damage the board. If shipping to shows or events I suggest only plexiglass or acrylic. No, it does not look as good, but it does protect it and when the work comes home (or is purchased) it can have museum glass reinstalled. If you must ship it with glass always tape the glass so that if it does break it will not shatter all over the artwork.
So glass or no glass is certainly up to you, but for this artist all of the work purchased from me will be behind museum glass maximizing the ability to see the details of your board, but also protecting the work from UV rays and incidental damages.