Why are my ocarina's high notes flat?
An ocarina's high notes may be flat for a number of reasons:
You may not be blowing hard enough
If your high notes are flat, there is a good chance that you are not blowing hard enough. As holes are opened, air can escape the chamber and the high notes become increasingly airy sounding. To compensate for this, ocarinas are tuned with a pressure curve that ramps up towards the high end; you may be surprised just how much pressure they need relative to the low notes.
Using a chromatic tuner or your ear, finger one of the high notes and vary your pressure, gradually raising it until the note sounds in tune. While doing this, the note may squeak before it reaches concert pitch. If this is a problem, see the next two points.
You may be playing in a cold environment
The pitch of all wind instruments is affected by temperature: they play flatter when cold and sharper when hot. There is a limited range wherein this can be compensated for with breath pressure, but doing so causes the shape of the breath curve to change. If you play using the same breath curve in a colder environment, the high notes will be flat relative to the low notes. You can compensate for this by raising your pressure toward the high notes. But in extreme cases, doing so will cause the high notes to squeak. Please read 'Playing ocarinas in tune in warm or cold environments' which covers this in detail.
You may be shading the finger holes
Even if a finger hole is open, there is a notable distance where the presence of a finger will 'shade' the hole, causing the ocarina to play flatter than it should. Watch yourself playing in a mirror and make sure you are not doing this. To check how the high note sounds under optimal conditions, play them while holding the ocarina by its ends so no fingers are close to the holes.
Be especially careful with this if you are coming from a tubular wind instrument. While the ocarina's fingering system is superficially similar, hole shading has a much bigger impact on the ocarina than it does on tubular instruments. You have to move your fingers a good way from the holes as a slight cumulative shading by multiple fingers will cause the high notes to play flat.
You may have a poorly tuned ocarina
As an ocarina's pitch depends on the total area of open holes, tuning errors are a big problem. If one hole is too small or too large, this will affect the tuning of every note higher than it. With glazed ocarinas, this can happen pretty easily as ceramic glazes have a notable thickness, so stray glaze in a hole can be a problem.
An ocarina's tuning can be checked by measuring the tuning of two adjacent notes, where they are both blown at the same pressure. Using a chromatic tuner, blow a lower note and, without changing your pressure, raise the finger for the next note in sequence. The higher note will fall flat. Begin at your ocarina's lowest note and check them all sequentially; their flatness should be fairly consistent but may increase or decrease gradually between sequential notes. If there are significant arbitrary variations, then you probably have a badly tuned ocarina.
See 'Identifying playable ocarinas' for more tips.