You have not given very much information regarding the presepctive from which you come-
If we are talking about ashing techniques for metals-then that would depend on where the formation of sulfates is important to your analysis:
Dry Ashing: Performed by placing the sample in an open inert vessel and destroying the combustible (organic) portion of the sample by thermal decomposition using a muffle furnace. Ashing temperatures usually range from 450 to 550oC. Magnesium nitrate is a common ashing aid. Usually, the sample is charred prior to muffling. Charring is accomplished using an open flame (crucibles/clay triangles/Bunsen and/or Meeker burners under a hood).
Wet Ashing: Samples are treated with a small to moderate amount of sulfuric acid prior to charring. The charring is performed using an open flame (crucibles/clay triangles/Bunsen and/or Meeker burners under a hood).. Liquid samples have a tendancy to foam. After the excess sulfuric acid is driven off, the sample is muffled from 450 to 550oC.
For example: Charring and ashing can form metal oxides. The objective is to keep the temperature on the low side (approx. 450oC). Metal oxides can be very difficult to dissolve.
Ashing products are often dissolved and subjected to further analysis such as ICP and low recoveries can be a result of incomplete ash dissolution. Sulfating the char forms metal sulfates. Typically, you could use a 1:1:2 mixture of HCl:HNO3:H2O to dissolve the ash.
Advantages: Little loss of volatile minerals occurs because of the lower temperatures used, more rapid than dry ashing.
Disadvantages: Labor intensive, low sample throughput. Typically, a digestion takes from 10 minutes to a few hours at temperatures of about 350oC-450oC. The resulting solution can then be analyzed for species of interest.