Melt the butter in a saucepan and warm the cooked, chopped bacon in it. Add the onion, garlic, thyme and bay leaf; sauté until the onion is translucent, about 3 minutes. Stir in the flour and cook for 2 to 3 minutes to make a roux.
Whisk in the clam juice and milk and bring to simmer to let thicken.
Add the potatoes, and clams and simmer for 10 to15 minutes, until potatoes are cooked.
If the chowder is too thick, stir in the additional milk or cream a little at a time until it reaches the desired consistency. If too thin, simmer gently a little longer, stirring frequently.
Remove the bay leaf and season to taste with salt and white pepper.