I (JM) would recommend thermal fracture if you are comfortable with a torch.

Procedure

1. Place package on a temperature resistant surface
2. Heat package with a propane torch until hot, maybe 5 or 10 seconds
3. Gently splash or sprinkle water on the package. This should create micro-fractures and make it easy to shear
4. Shear the package open by applying light force. Unlike a non-fractured package this should be possible using simple pairs of pliers or by hand

Tips:

• Chip does not have to be heated until red or white hot. Heating this high increases risk to damage the die from glass reflowing as well as general thermal stress
• Do not throw the chip into liquid or move it all while it is hot. This can cause the glass to shift and get onto the die
• PDIPs should be supported (ie with a block of metal) as the leadframe will become very weak
• If it does not shear open the first time try again. Consider increasing heat, pressure, or simply do more runs

Advantages:

• Can be very clean and low risk
• Tends to be pretty clean and not throw a lot of debris onto the die

Disadvantages:

• Thermal shock can potentially also break die
• High temperatures can warp die
• The die is exposed but still in the package

Chip before (both identical):

One of these was thrown into water and the other had water dripped on it. Comparison:

Dunked chip close-up:

Dripped on chip close-up:

The motion of throwing the chip into the water caused the glass to flow. Additionally, it isn't as regular from moving around and so didn't break nearly as cleanly.

Advantages

• Can completely remove a die

Disadvantages

• Removed dies still may have an uneven bottom. This can be solved by very carefully heating them on a surface that glass won't stick to until the glass melts

The glass tends to be much weaker than the ceramic. Therefore, its possible to shear a chip open by moving the top and bottom half in different directions.

Procedure

1. Place bottom half of package into vice. It may be desirable to put a metal space in between the pins to keep pressure more even
2. Firmly hold the top of the chip and twist. Keep the movement orthogonal to the glass plane

Ideally this would probably be done using a custom fixture to hold the top and apply even and straight force

Tips

• Plumbing wrenches work well to hold the chip as their width can be adjusted for even pressure while still holding it tightly
• Be careful with long CERDIPs that you don't apply too much torque on an end and snap the chip in half instead of shear it

Advantages

• Very readily available materials
• Does not use any consumables

Disadvantages

• Throws debris onto die
• Can be difficult to effectively grip top half
• Can be difficult to tell how much force is required
• Unpredictable how it will shear

I see no reason why you couldn't grind one open (eg with a Dremel) but it would probably take a long time and would require extensive cleanup after.

Might provide some hints if we knew how these were produced