There are many types of ceramic so the results can vary. Typically there is some material removal from laser processing ceramic and thus an "engraved' mark is achieved. In some cases not only the top surface is affected, but also the underlying material. This is more common in commercial grade ceramic items such as earthenware.
Industrial ceramics are typically much more heat resistant and have a better quality front surface. Because of the surface condition, engraving is usually of the very highest quality.
In most cases ceramic materials are best processed with powers of 45 watts or more. This also allows for higher production rates due to the extra speed that corresponds to higher power.
There are special tiles that are readily available, that actually create a rich dark contrast, when processed with a CO2 laser. Trotec will be happy to provide vendor information related to these products.